Can I make a store product available only to certain grades?
Yes. By default, every product in the store is available to everyone. For products that only make sense for some families — a 5th grade field trip, a kindergarten supply kit, an 8th grade graduation package — you can limit the product to one or more grades so that other parents don't see it in the store.
Open the product's edit page from Store/Manage Products and find the Grades section. Check each grade the product is for, then save. To make the product available to everyone again, uncheck all of the grades.