School's in for summer. Jack Whitehall's Bad Education is due back soon on BBC3 and Greg Davies has a class act, Man Down, coming on C4. But first out of the changing rooms is Big School, which starts on BBC1 tonight and stars David Walliams and Catherine Tate.

So is it gold star or detention time? David Walliams, who co-wrote it with The Dawson Brothers, plays Greybridge School's science teacher Mr Church, who is immediately smitten by Catherine Tate's pouty but innocent French teacher, Miss Postern. Making up the love triangle is central casting track-suited instantly recognisable PE master Mr Gunn, played with considerable Gene Hunt-style unreconstructed relish by Philip Glenister (nice of Walliams to give him the best lines).

The cast oozes quality and Baftas by the bucketload from soup to pudding. The first episode also introduces Daniel "Irritating stude from the BT ads" Rigby as desperate-to-be-cool Mr Martin – "Mozart is totally Yawnsville" – who is not a million miles away from the teacher Walliams played opposite Vicky Pollard in Little Britain, which itself was not light years from the cool teacher played by Stewart Lee in Fist of Fun.

There's also Joanna Scanlan from The Thick of It/Getting On as Mrs Klebb (bit of a From Russia With Love nod there), Julie T Wallace who barely has a line in episode one, and Frances De La Tour as the hard-bitten head Ms Baron. So the line-up is pretty much grade A and more than capable of turning the flatter written-on-a-fag-packet bits of the script into something a cut above the banal.

Big School is an interesting animal. It is unashamedly mainstream, but there is no studio laughter. It is not Miranda or Count Arthur Strong. On the other hand, thank the lord, it is not The Wright Way or Mrs Brown's Boys either. Yet Big School is defiantly old school, not in any way related to The Office or Alan Partridge. It is not broad, not filthy (there is one mild erection gag and a sheep-shagging quip) and, while not a comedy drama, it is plot-driven – the main narrative arc looks like being Walliams' fumbling romantic pursuit of Tate. It is attempting to crack one of the toughest nuts – a totally nation-uniting comedy – it has to be judged in that context and it is not a bad stab at the almost impossible.

Greybridge, of course, resembles no school I've ever seen. Not even the one in Please, Sir. The kids are absolutely silent in assembly until addressed by Miss Postern. There are no lippy Vicky Pollards, maybe a hint of Tate's Lauren in the pupil asking "why is you speaking foreign?" when Miss Postern starts talking to her class in French, but that's virtually the extent of their misbehaviour. The one boy who has detention leaves early and there appear to be no recriminations.

The shock of Big School is that it plays it so safe even though it is after the post-9pm watershed, though I guess that's BBC1 on a Friday night for you. I loved David Walliams from the time he had bit parts in sitcoms such as Spaced to his deserved success Little Britain. Since then he seems to have embraced a full-on tabloid-friendly mainstream showbiz life. He hasn't lost the ability to be funny and there are occasional glimpses here in a look or a walk or a subtle pause, but I preferred him when his wit was more warped. Unless he loses interest in Miss Postern and ends up in the stationery cupboard with Mr Gunn in episode six this looks like playing things fairly straight in every sense.

Catherine Tate is very good oozing a strange kind of innocent sexiness. Her career has also gone off in all sorts of interesting directions since her breakthrough sketch show and it is good to see her back doing proper comedy (she is also currently making an English version of Everybody Loves Raymond with Lee Mack). And Walliams and Tate work well together. There does seem to be some chemistry for the science teacher. 

It is never easy to judge a sitcom from episode one and, as Ben Elton has shown, it is never easy to write a mainstream, crowdpleasing sitcom without resorting to clunky cliches. Hopefully in the next five weeks the glitzy supporting cast will get to do more. At the moment the report reads "a lot of potential that needs to be realised…could do better". 

Big School is on BBC1 tonight at 9pm.