don't bring money to school, that way you can't buy food from the vending machines (stick an apple or something healitju for if you get hungary after school. For lunch don't have any money in that account so you won't be tempted either- bring healthy stuff- a turkey sandwhich,veggies, fruit, water. Just take the temptations away. For exercise it doesn't have to take a long time, just jog for 20 min sometime.