In theory, yes, but your working hours on school days are very limited at 15. Supermarkets tend to like people to do full shifts, even if that's only on one day per week. I doubt a full shift would be allowed on top of a school day.
You might be better off looking for a smaller shopkeeper to employ you after school.
Not sure actually. I think most supermmarkets won't employ you until you are 16, and problem is now that a lot of 'after school' type jobs have moved to night shift as openig hours extend etc.