if the pipes are cast in the foundation, you will get one hell of a blast out of all the toilets in the bathroom...if the pipes aren't, it will probably just break the pipe.
also, you would need one large firecracker, like an M80 or a quarterstick...a bunch of small ones wouldn't work.
fair warning, they would probably be able to get you for detonating an explosive device on school grounds...improvised, if you made the M80...I hear federal is lovely this time of year.