Well, I just got a little miffed, probably should have done more homework. Toured all the LBSs in my area, came home with a set of pannier bags. Big enough, between them, to hold all my 11yr son's schoolbooks, notebooks, school supplies, etc. Stuff that I used to leave in my school locker 40 years ago, but his school doesn't have lockers, don't ask me why.
Full backpack weighs maybe 30 pounds. Was tasked by family CEO to design or buy something that will allow him to put this weight on the bike (Trek 7200) instead of on his back. Brought home the set of pannier bags, looked to me like a good solution. Good weight distribution, strapped tightly enough to the back rack, and when he got to school he could undo two straps and lift the panniers off the bike and carry them around. And on the 3 days a week he has band, he can strap his trumpet on top of the rack as usual, since the panniers leave the top of the rack clear. Son took one look and said he didn't want to carry the panniers in school because everyone else has backpacks and these look weird.
CEO and I just finished going over possiblities. Most likely one seems to be a backpack that can also hook onto the side of the back rack, like a single pannier bag. Trumpet can still go on top, or on the other side somehow if balance is an issue. I can ride it unbalanced like that, but I'm considerably bigger than son.
I could modify his present backpack with hooks, and probably a board inside (like the pannier) so cloth wouldn't sag into the spokes. With a strap lower down of course, to fasten near the axle like the pannier does.
Before swinging the shop into action, it occurred to me that I'm probably not the first person to have this prroblem. Maybe there's a commercially-produced product, available in bike shops (though I didn't see one in today's LBS tour), which is a bag that can hang on the side of a back rack, and then be taken off an worn like a backpack? And big enough to hold all those book, stuff, kitchen sink etc.?
Does anyone know of such a backpack-that's-also-a-single-pannier?
A single side basket is also a possibility, that he can drop the filled backpack into.
Alternate responses, such as Tell the son he can use the panniers or walk to school with his books in a roller backpack, have already been considered and rejected, though some of the votes were close. I hope to explore this single-pannier-backpack idea now.