I've had one since kindergarten.
Why do people complain about using backpacks? For forgets sake all they do is make your life easier.
Because, kids can easily carry beepers, compact game things, cellphones, knifes, guns, bombs, and WMDs in them, bring them to class and BOOM 5th period English turns into a massive crater.
Besides that, most of them are obstructive. They take up space and kids aren't careful with them. They fool around, hit into each other with them, people can easily trip over them when they are left somewhere, they can easily get caught in doorways (Leaving you stranded forever!), and a number of other reasons. Just carry your books. Plus, you put a lot of strain on your shoulders and break circulation to your arms when you carry bookpacks. Schools don't need armless kids walking around and feeling discriminated because they can't support backpacks anymore.