If you look at the classic comedies over the years, there seem to be a number of common themes - in particular a number of strong and contrasting characters (even though there may be a strong central character - Basil Fawlty, Risgsby, Del Trotter etc.) and following a situation that most of us wouldn't really want to be in (either because we've been there and don't want to go back or because the sheer idea of it gives us the shivers) - after all, who would really want to do business with Del Boy, live in one of Rigsby's bedsits or stay at Fawlty Towers?!
Big School is in a similar vein - a strong cast of dispirate teachers who we almost certainly wouldn't want teaching our kids (but who we probably recognise to a greater or lesser degree from our own schooldays)! Other reviewers have highlighted some stand out moments so I won't repeat them here (although I have to add the methane chemistry experiment to the list of favourites) but suffice to say that this was outstanding comedy.
However, Big School is a bit of a guilty secret - you wouldn't sit down and watch it with your granny and, equally, watching it with your kids may result in having to answer some uncomfortable questions. For that reason, don't open the package until the kids have gone to bed and then indulge yourself. Not everyone will appreciate the humour but as long as you're the sort of person that has a little snigger at things they shouldn't from time to time, you won't be disappointed.