lol, probably because it's against the rules. If it was allowed, no-one would do it!
I remember on my last day of school (when I was 16 - so the last day before we moved on to the 6th form the following year.) The teachers knew that on the last day, the students were going to be up to no good, lots of pranks etc. Anyway, they weren't wrong and at 9:30 am there were already 2 students totally drunk having downed a load of beers lol ! So anyway the teachers called us all in for an "emergency meeting" at 10 am to lay down the law a bit.
And they actually proposed an amnesty deal - The deal was that if we had anything in our bags that was against the rules / for pranks that we had to hand it in right now by putting it in one of the plastic containers at the front of the assembly hall on the way out. Anyone doing so would
not be punished, but if anyone was caught afterwards with anything against the rules, then there would be big trouble.
So on the way out I had a quick look in the container, and there was all sorts of stuff in there - bottles of vodka, whisky, cannabis, knives, batman costumes, red paint, p**n ..... The teachers were like W T F ?