Is there a younger sibling, cousin, or neighbor who could use it? If not, a local homeless shelter or other local charity might accept gently used school supplies. You can also offer it for free via Freecycle or your local Craigslist. I think that a little investigation will offer many options for donating school bags in your area.
But what if the bag is not in great shape? Even it’s had a lot of wear or the straps are broken, there are still options for reusing an old school bag.