Hi guys, I'm in upper sixth and thought I would just give my take on this.
I went to a pretty rough secondary school with over 1500 pupils. Drugs (Pretty much just weed) could be got very easily, almost all "chavs" smoke weed (Not only chavs smoke weed however the non chav ones arn't stupid enough to mix weed and school). The problem schools have with dealing with it is that the chavs don't care what teachers/the school says to them, and for a big secondary school it is pretty difficult for them to kick someone out because they would have to find them somewhere else to be educated. Thus it is very difficult to deal with the problem.
TBH drug education did nothing in schools, no one payed any attention, and if you did pay attention and were reasonably intelligent you could tell that it was blatant propaganda (Very very dubious "facts" and a very one sided view that pretty much centered on "drugs are bad mkay" kind of view (Bloody hell a southpark reference).)
Yeah I agree with the irony about how schools seem to punish things disproportionately. Swearing at a teacher and storming out of class would get you an afterschool detention, however some teachers would give the same punishment for simply talking in their lesson or not handing in a piece of homework.
The school won't tell parents because it will put the school in a bad light, tell parents all the good stuff but not all the bad stuff. My parents were also shocked when I told them about all the stuff that happened (teacher locked in cuboard for 2 hours, many holes punched in walls, ect ect.)