yo don't do anything that could get you kicked out, no point ruining your life for a few quid.
buy a small fridge, a lean mean fat reducing grilling machine, then loads of bacon, bread, ketchup, mustard kettle, tea coffee and soft drinks. then late at night sell bacon sandwiches for a profit. by then everyone's starving and school food sucks anyway. get it in bulk for cheap then sell it on. do the same with sweets and crisps and you can make a decent profit. just make sure you have a kitchen otherwise your room will smell of bacon. you could also get burgers, steaks and other meat all in sandwiches.