Heavy school bags, insufficient lockers for the number of pupils, insufficient time to dash between classrooms at lesson change, all were problems when my DC were at school and continue today with the DGC.
When I was at school aeons ago, the teacher came to our classroom, apart from for certain lessons eg Science which were held in the laboratory. Books etc remained in our own desks apart from those we carried ome for homework.
Our GP called it "Paperboy's Back".
Children can have growth spurts which can mean the bones have not fully developed from cartilage, cartilage can be bent resulting in pain and sometimes permanent damage to hips, shins and backs.