We had lockers at my secondary school. People would it put their sports kit in them, and nothing else. Out school was pretty big, with 5 buildings, each for a different subject area (humanities, sciences + maths, design + technology, sports hall, and the artsy drama "other" building). Lockers were only found on the ground floor of three of the buildings. It would have been really impractical to keep everything in there, collect it at lunch or break (with a third of the school crowding into each buildings' ground floor at once) then potentially trek to another building where your lesson is

Also, think how annoying would it be to carry in your hands a few lessons worth of books and a pencil case and whatever else you might need, when a bag would make everything easier