I’m viewing it from the perspective of age and purpose. At that age, the school bag is more a vehicle for ‘homework’, ‘art’ to transport safely home, lunches, classroom school supplies contribution, and the ‘Agenda’ used to communicate between teacher and parent. The parent typically packs it. The child “unpacks” upon arrival to school and passes along the Agenda to the teacher for response, etc. and puts their lunch box in their cubbie. The teacher will pack back the Agenda and i.e., birthday invites. The ownership is different. There is an unspoken ‘boundary’ of importance. Until the child gets a new backpack and the old one is repurposed for i.e., going to the pool in the summertime. From the first day when it is shopped for and chosen, the child understands it’s for school. That’s how it rolled when my kids were growing up.