Main problems that can come up while setting a snack shop at a school can be the following.
Students at school usually don't have big amounts of money, they cannot buy the snacks that are high in price. So low priced snacks should be there that the students can afford.
Low priced snacks are usually unhealthy. So Government won't allow to put those snacks on the shop. So the snack shop owner should put snacks that are healthy as well as low priced.
Secondly, students buy snacks during the break time. So large number of students come and ask for food in a limited time period. Snacks have to be ready just before the recess to meet the students' orders. So such snack shops need space to store those food items.
Thirdly, there is no margin of profits in such shops. So snack shop at a school can't be persuaded as a business.