This is not Health and Safety which is important, even vital to the well-being of industrial workers, builders, farmers and members of other dangerous occupations. They are concerned with preventing avoidable accidents with heavy machinery, sharp blades, scaffolding erection, vats of boiling liquid, acid or caustic substances, etc. which can and do kill and maim. They have not banned flapjacks of any shape or size any more than they banned children from playing conkers unless they were wearing safety glasses. This decision was taken at school level, presumably by the head. It is, if you'll excuse the pun, pointless to ban triangular flapjacks on the grounds that they are dangerous when thrown but still allow square and rectangular ones. Ban food throwing instead.