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.
Does he really need all of that stuff? Rain jacket & pants? Most high school kids are OK getting soaked. Flat repair & pump? How long is his ride? Can he walk in the unlikely event he gets a flat? Buy lunch @ school?
Notebooks, textbooks, paperback, and a computer? Talk to your school. They should be telling students what books they need in class and what to keep at home. Many students here carry only a notebook or iPad back and forth along with maybe one spiral notebook. This is very intentional due to concerns over how heavy backpacks were getting and their causing back problems.