"weed" rules are hardly relaxed in school to my knowledge in the uk.
I know one person that got suspended and another that got expelled for having it at school.
The school is allowed to do a locker search no problem, however if they really wanted to do a search, they aren't legally allowed to, they have to get the old bill in.

Then again, it's swings and roundabouts, this is quite a few years back, and what with it going down to c class maybe so, also, how would they justify a search, unless the kid was a punk and got caught (which they both did)