Comedy relies upon jokes, character, timing, misunderstandings, the suspension of disbelief - even banana skins. There is a huge bag of tricks in which the skilled comedy writer can rummage. Not on BBC1, though. A BBC1 comedy requires nothing more than familiar faces: just get a bunch of famous people on board and job's a good 'un.

But, wait. I'm being too harsh. Big School (BBC1) did make use of one comedy trick because viewers were indeed required to suspend disbelief: we had to imagine Catherine Tate as a luscious, desirable woman instead of that nattering middle-aged schoolgirl she always plays. Massive suspension of disbelief was needed, and not just suspension, more like a moratorium, a termination, a colossal extermination.

Catherine Tate plays sexy French teacher Miss Postern at Greybridge School. The awkward Mr Church (David Walliams) fancies her, but so does sweaty Gym teacher, Mr Gunn (Philip Glenister). If these names aren't famous enough to invigorate the anaemic comedy, we also have Frances de la Tour as the wine-glugging Headmistress and Joanna Scanlan as Mrs Klebb.

The pupils don't feature in this school comedy except as selfie-snappin', Nando's-lovin' teenage caricatures. Instead, the focus is on the staff who, hilariously, are more silly and immature than the children. One of them, Mr Martin, is so foolish he announces he's resigning to become a pop star. His debut single is called Soar, which permits far too many limp jokes about his single being 'sore'.

Being a school comedy, jokes about spelling are supplemented with jokes about pronunciation. Mr Gunn, the Gym teacher, is stepping in to teach Geography but - and brace yourself here for the merriment - pronounces it jog-raffy. He does this several times. Why? Who knows? We were fed this recurring gag which was based on nothing other than the mispronunciation of a standard word. It wasn't a double-entendre and it wasn't a tongue-twister. It was just a man saying 'geography' wrong.

So, jokes about spelling - TICK. Jokes about pronunciation - TICK. What's next on teacher's checklist? A joke about grammar, maybe? The old 'dangling participle' is surely screaming out for a pun but no, they opted for jokes about the mentally ill.

I have no problem with jokes about mental illness as long as they're funny. Laughter is, after all, the best medicine, but Big School approached mental illness as though they were playing to an Edwardian music hall crowd, encouraging them to fling peanuts and shoes onto the stage at the manacled loony. A teacher has been sacked due to a breakdown - perhaps this was explained in the last series - yet he appears in the staff room, chirpy and tucking into the biscuits. He is played by a fat man. A fat, bald man. A fat, bald man who is gorging on biscuits. Look at the mentally ill person! Mr Church leans across. 'How's your lunacy?' he asks, loudly. This might be funny if the joke was on society's awkwardness towards mental illness, or Mr Church's general lack of social graces, but the loony had already been set up as fat, bald, cumbersome and greedy and so the joke was squarely on him. Mad people are funny, see. Throw more peanuts at him before he's manacled and led away.

Subtlety must be forbidden on BBC1, or maybe they worry they only have a 30 minute slot and subtlety would take too long to filter down to us, so whack, bam, splatter - here are your jokes. Quick! Laugh at the loony before the credits roll.

The crazy teacher had returned to school in a new role as caretaker, so he shambled around in overalls, emptying bins, being ordered out of rooms, and eating stale buns. If the freak show flavour wasn't quite strong enough, he poked his head through the canteen window at one point and begged for food like a whipped dog. Mr Church fed him Fish Fingers through the gap, but only after the loony begged for it to be slathered in ketchup. Then he slobbered it down like an animal. The only thing absent from the scene was Frederick Treves.

Everything in this appalling comedy was a cliche. The sexy teacher was sexy. The creepy teacher was creepy. The posh, awkward teacher was posh and awkward and the loony pawed at the window, wanting scraps dropped into his mouth. It made me think of When The Whistle Blows, the fictional sitcom of Ricky Gervais's Extras, where Andy Milman bemoans the BBC's need to have his characters adopt silly wigs, glasses and catchphrases in order to be 'funny'. No-one can be nuanced or real. There was even a scene which was almost copied straight from Extras: Miss Postern is challenged to say something in Mandarin. 'Chee wong yo wong chee,' she says, or something to that effect. When The Whistle Blows has a near-identical scene with characters bowing and yowling in fake Japanese but, and here's the massively important point, Extras was doing this as satire: mocking the lack of subtlety in broad BBC1 comedies. Perhaps if Big School was broadcast on BBC2 we could regard its humour as mocking but, no, it has the big, bland BBC1 stamp on it.

There's nothing dishonourable in drawing influence from your comedy betters, but Big School seems to have taken lessons from the wrong sitcom. They've aped When The Whistle Blows but without the encircling brilliance of Extras it just looks like they've copied the answers from the wrong jotter.