You could work upfront as a cashier or in the photolab until you graduate. As long as you tell the managers(upfront and pharmacy) that you want to work in the pharmacy when you graduate high school, it should work out for you. You should stay in contact with the pharmacy department while you're working upfront so that they don't forget about you 😛.
If you don't like the store that you start working at, try to get a transfer. Don't expect anything to happen over night, but they should transfer you if you're unhappy. Things will change in the pharmacy when different people work there, which could be good or bad.
Good Luck!