If you have a high schooler bike commuting, what do they use to carry stuff?

My son's bag is not large enough. It is a medium or smaller messenger style backpack. By the time you put in some notebooks, a couple textbooks, laptop, calculator, pencil case, flat repair stuff (tube, etc), mini pump, Ulock, rain jacket, rain pants, paperback book, lunch, snack, wallet, phone, etc - it is not large enough.