Make sure that the backpack fits your kid comfortable and that the backpack gives them good back support. This is very important because all the things they will carry around can cause strain on their backs which can be very bad for them in the future.
Take some heavy books from home, or something while you are out shopping for the backpack and test each one you find. Ask your kid how they feel, if it hurts or if it’s comfy.
And remember to ask about the straps–if they dig into your kiddo’s shoulders, they can bruise…not fun (this is what happened to me in high school).