Riding w/ a heavy pack can cause medical probs so do be careful of that.
In Europe (NL, DE, DM, SE particularly) kids wear light packs but their bikes always have a rear rack for heavier stuff. The racks have attached Bibia straps that make it quick and easy to stick a pack on. Some kids have an old wine crate bolted to the rack and occasionally you'll see panniers but these latter are less popular since they prevent friends from riding on the rack.
One note, K-12 kids in many EU countries never (Finland) or rarely (NL, DM, SE) have homework and yet score significantly higher on PISA tests than U.S. students.