Is it in the school rules that they can confiscate smoking equipment? Cos if it is'nt written somewhere, in black and white (on a contract that YOU and a teacher signed, with the teacher witnessing it aswell) that can be classed as theft. As long as there is'nt any resin left in there, then they are breaching certain laws.
Remember - it's YOUR propery, YOU legally own that bowl seeing as YOU bought it with YOUR own money (or someone else bought it for you... as long as it's yours, then demand it back).