Being stoned in public is fine, the only way any could really tell is if they stare hard at your eyes for ages, and who the fuck does that in the street (except for townies but their usually high on A-class drugs themselves).
Stoned in school is a biggun though, I dunno about your particular school but I can imagine it'd be a lot easier to notice. You should make your own plan for staying sly in school. At college, noone gives a shite !