Greg Armstrong is teaching - and he's completely naked. A group of first year students are gathered round him, watching, while he takes a shower. He's sluicing off the flour and water paste that had been tipped over him almost an hour previously, and he's lecturing from under the hot water. This is the first time I observe him working. It's the end of September, the start of the new academic year, and this lesson - part of the induction course - is on how to de-gunge oneself.

'Just look,' he says, lathering his crotch and picking up a comb, '- there's no problem. It'll come out easily.' Despite the shower, the mixture's residue has speckled Greg's abundant black pubic hair with dough-y, dirty white bobbles. A bucket has been poured down his underpants and the gluten in the flour has coagulated.

'Watch,' he says. 'You must always comb in the direction your hairs naturally lie,' and his students give this practical demonstration their serious attention. This is all part of Greg's job. His official title is 'Teaching Assistant', but there can't be many teaching assistants who routinely invite their pupils to stare intently at their genitals.

'As Graham mentioned,' Greg says, referring to his colleague who had inflicted this damage on him, '- flour and water paste is probably the most difficult of all the gunge-y substances you'll ever encounter, so if you can cope with this you can cope with anything.'

He seems marvellously relaxed about being naked in front of his fully-dressed students. To be frank, he's not an especially well-endowed young man, and when he pulls back his foreskin to pick off a coil of paste that has lodged around his glans he only draws focus to his stubby penis. 'Stuff gets everywhere,' he quips. 'Any questions?'

An Asian student, who had been visibly wincing when Greg had scraped the comb over the underside of his scrotum, sets off titters by asking: 'Does it hurt?'

'Erm ...'

And the nervous laughter builds while Greg hesitates.

'- Yup, I'll be honest, you feel it a tiny bit. But like Graham's just been telling you, that's all part of being a slapstick clown. Putting up with discomfort is what this game is all about.'

I'm at The National Academy for the Training of Slapstick Performers, the college in Harrow, Middlesex, which offers practical instruction for circus clowns, panto comics, performance artists, comedy stuntmen, mud-wrestlers - indeed for any performer whose work involves them getting messy.

It opened in 1984, in the purpose-built premises it occupies today. The Academy is for men only, and is widely known as 'Nats-pee', after its acronym, or often simply 'The Slapstick School'. There are classes in everything from traditional circus and panto routines, by way of contemporary messy physical theatre (think La Fura dels Baus), to night-club cabaret mud-wrestling. A variety of skills are covered over a 2-year course: how to throw a custard pie; how to receive one; how to sit in the stocks at a fairground and take all that's coming; students are instructed in techniques for aerial work (because swinging upside down from a rope is a frequent demand of avant-garde spectaculars); they're taught tumbling, gymnastics, even high-diving; but most importantly they gradually acquire the stamina and fortitude that's needed for getting messed up day after day.

Earlier in the morning, the new intake - including the group of thirteen who'd been watching Greg's ablutions - had attended a lecture in the main rehearsal room given by the academy's principal. They entered to see that the dais was covered with a tarpaulin, and that on it a table was loaded with transparent plastic buckets of water and opened packets of flour. There was an immediate buzz. This was going to be their first practical demonstration.

Students can enter from 18. In theory there is no upper age limit, but the course is physically demanding and it's rare that anyone is accepted who is older than 39. Closer inspection of these freshers reveals that they're various shapes and sizes - two of them are even on the dumpy side - yet on first sight they'd looked like clones in some sci-fi dystopia because they were dressed identically in white cotton boiler-suits. Students are not allowed to wear their own clothes for classes. There aren't many rules about which this otherwise relaxed institution is emphatically strict, but this is one of them. Every day as they arrive, students must shower and change into the Academy's compulsory uniform: the overalls, jelly sandals and regulation underwear, all of which are communally shared.

Graham Weir, the college principal, enters. He's lean and energetic in a yellow track-suit, looking much younger than his 46 years. He has a recent PhD in Theatre Studies, yet in previous lives he was a circus clown and stuntman (it's him who doubles the jaw-dropping fall in the 90s British film comedy Running on Empty when the hapless marathon man gets tipped down an embankment on a Tesco trolley into a cesspool).

Following him into the room is Greg Armstrong, wearing a boiler-suit similar to the students', except that his is bright orange.

'Good morning, gentlemen,' Weir says, commanding instant silence. He pours water into a bucket that is already half-filled with flour, picks up a long-handled spoon, and begins beating the mixture into a paste.

'Flour and water,' he intones. 'Beautiful. What they filled the custard pies with in the silent film comedies a century ago. The original slapstick material - cheap, versatile, and disgustingly sticky.' He allows a gobbet of the paste to plop off the spoon.

'In a few moments time,' he continues, pouring some of the mixture into a pre-baked pastry case, ' I'm going to slap this pie into Greg's face. As you know, Greg is a member of the teaching staff, but he is going to suffer the humiliation of getting messed up in front of you all, and he will do this because he's a trained slapstick professional - just like I hope you will all become. Yet before I do this, I want you to imagine how you would feel if you were in Greg's position right now.'

A semi-articulated murmur, more like a sigh of desire, briefly swells through the room.

'Okay,' Weir says, shushing them, '- if you think you'd like to be in Greg's shoes, you've made the right decision. You're in the right place.'

Later, Weir tells me that in just a couple of weeks time these first year students will have learnt a simple slapstick routine that gets each of them mucky, and that they'll be performing it every day at the end of classes.

'From an early stage it's good for them to get used to at least one daily messing-up,' Weir explains. 'They've maybe had the odd experience of getting mucked at some one-off charity event and thought it a laugh, but they soon find out that being messy day in, day out takes real commitment - and resilience.' He is used to a high drop-out rate. Of the annual intake of 39, maybe not even half will finish the course.

He walks upstage of Greg with the overflowing custard pie and holds it millimetres from his chin, long enough for tension to build. Suddenly he tips it through 90 degrees and mashes it hard, squarely, into Greg's face.

Greg scarcely flinches. He keeps his eyelids shut as the lumpy, viscous goo crumples over his forehead and cheekbones, and slides in a curtain down onto his upper torso, where some of it drips into the V of the unbuttoned top of his overalls, commingling with his chest-hair.

This is the sort of treatment Greg can expect every day for the duration of his contract throughout the academic year. His title might be 'Teaching Assistant' but his role is that of stooge. Every time one of the lecturers wishes to demonstrate with a real live victim some particular move or technique for messing-up, it's Greg who'll be taking the hit. For instance, the next time I visit, Greg is demonstrating different ways to tumble into water, falling repeatedly, fully clothed in his orange boiler-suit, from off the top diving board of the academy's indoor pool.

The teaching assistant job is traditionally given to a recent alumnus, and Greg has had a year since leaving 'Natspee'. His intervening time in the wider world has been pretty typical for one of the academy's graduates. He got a job in panto in The Midlands, playing the back end of a cow, but also performing: 'one of the messiest decoration scenes you could ever imagine. By the finish there was so much paste we could slide through it on our stomachs from one side of the stage to the other. I loved it!'

Then through the recent summer, Greg was touring in a van all over the country with a small company that gives short slapstick performances outdoors at fairs, fetes, and county shows. Four, even five times each day he was putting down the tarpaulin and mixing the gunk, then queuing after the show in his slimy costume for the portable shower and its dubious recycled water. 'Brutal!' he laughed. 'Fun, though.'

Greg is being put through equal hardships in his present job. As the pie-paste continues to slip down, Graham Weir tells him to take off his overalls. Obediently he strips. A groan goes up in the lecture hall the instant he reveals his underpants, a mixture of scorn and sympathy. He has on the same regulation underpants the students themselves are obliged to wear. Only a day ago they were introduced to this compulsory uniform, and it's clear they think it hideously uncool. True, these underpants are old-fashioned and not obviously sexy: they're high-waisted, classic white Y-Fronts made from the hole-y material known as cellular cotton, or Aertex. Later when I ask Graham Weir why these?, he tells me they're cheap - a necessary consideration given their multiple messings and washings and consequent short life-span - but also that it's good for the students to have their dignity undermined a little, even by their underwear. 'A good clown should expect to look ridiculous,' he says.

Greg has learnt this lesson well. He just stands there in nothing except these unflattering briefs, heedless that he's giving us a free display of his chunky physique, as well as his tiny jutting packet.

Graham Weir tells his students that the face is the primary target in slapstick, then asks them which they think the secondary one is. He gets:

'The head?'

'The bum!'

'His hair!'

'The crotch?'

'Tits and chest!'

'Go for his balls!'

- but he makes no comment. Instead he instructs Greg to turn round and bend over till he's touching his toes so that all we can see of him is his backside, the perforated material of his briefs stretching taut across his firm buttocks. Weir picks up a second pie and splats it viciously onto the centre point astride Greg's crack.

'The answer is the arse, gentlemen,' Weir pronounces. He leaves Greg in this demeaning predicament while he begins discussing the sacrifices that all slapstick performers - 'clowns', he calls them in shorthand - have to make with both their comfort and their dignity. Then he gets Greg to kneel down, facing us, and lean slightly backwards. Weir picks up the second bucket he's mixed. It's full to the top with paste. The students around me make 'Oh yeah!' noises when they realise what's coming. This time there's no dramatic delay, the action is fluent and sudden. Weir obliterates Greg's head with a swift upturn of the bucket and a splashy cascade of thick, weighty paste. It plunges down Greg's abdomen and is channelled to the floor between his thighs. As soon as Weir removes the bucket's rim from off Greg's shoulders we see the younger man open-mouthed and gasping through skeins of drooping paste. His powerful biceps remain clean, but his torso is entirely coated with an even sheen of sticky whiteness.

Weir gets him to stand up and hold the waistband of his underpants - now mostly sodden - a little way out in front of him.

'Think, gentlemen,' he asks his students as he returns to the table to pick up a second bucket that is also full to the brim with paste, '- how a slapstick clown differs from a comedy actor.'

He comes back over to Greg, and stands beside him, ready to pour the bucket's contents into the waiting target.

'The actor might have to play a situation which makes his character look like a fool, but he doesn't undergo any unpleasant experience himself. His own dignity as a person is never jeopardised. But a slapstick clown really does get that pie in the face, or that bucket of water over his head. It's not faked. It's real. The gunge makes actual contact. A clown's skin actually does get wet. He feels it, every single time. In order to entertain his audience he genuinely suffers, and he must be prepared to squander his dignity in doing so, too.'

As he's saying this, Greg is demonstrating iron self-discipline, standing motionless, still holding open the waist of his smeary underpants in readiness for the inevitable moment when Weir will tip the full bucket down inside. And when it happens, and his crotch is briefly distended outwards under the massy surge, it's the students who react with a 'Whoah!' and not him.

After this he has to stand for nearly an hour while Weir continues his introductory lecture, expanding on his theme. Every so often Weir turns and offers up Greg as the visual confirmation of his ideas. There Greg stands, vulnerably semi-naked, coated almost completely in flour and water paste, with slicks of it seeping down each thigh that have leaked from his gaping and sodden underpants. He's increasingly cold, sticky and uncomfortable, yet he's ready and willing to take any further amount of slapstick punishment if his boss requires it.

Once the lecture is over Weir dismisses the class, dispatching a small group into the shower area to witness Greg's cleaning-up process, and Greg at last moves, dripping and shivering, towards the door. You can tell by the ways their eyes follow his paste-spattered buttocks that there's not a student in the room who doesn't ardently wish they were him.