dabbled in micro-explosives - made glycerine in organic chemistry with a mate and surreptitiously "tweaked" it into nitro-glycerine.
Application on to one of the school boiler doors and a magnesium fuse.
It worked more than a treat - completely blew the doors off their hinges.
All hell as they tried to work out who did it.
Was even worse at Uni, the best being the phenolphtalein (the litmus tester for acids and alkalis). Careful calculation of the quantities needed (as it can be not nice stuff in excessive quantities, the "theft" of it and then addition to the communal men's hall coffee pot at lunch time......coffees and teas after and everybody going for a slash before afternoon lectures and practicals, discovering that their urine was coloured red.....
The doctor's surgery ("Fingers Lamont" for his predilection for touching up the girls) was en route to our afternoon labs. We dropped in and, by then, all we had to do was sign the book. The hall went bananas trying to find out who it was, immediately targeting those who hadn't been to the surgery......