It can be so humiliating not to be able easily to supply the things your own family needs. Charity - loving assistance - is great, but nobody likes being thought of as a charity case.
So... you may have to be unabashedly deceptive! If you can afford even a $10 GC for, say, Wal Mart or some other economical store, put it (with no note attached) inside their front door when they don't know - or mail it to them anonymously, disguising your handwriting. Sounds childish, but it works! (You'll have to keep it a secret forever.)
Or... you could go to the store yourself and "happen" to overbuy a little if you're getting school supplies yourself.
If you go through your pantry and find food your family doesn't need, call her and ask if she'd like some of it. If she says no, that's all right. If she takes some, you've just saved her some money she can use for school supplies.
The idea is to treat her like, "You're my friend," instead of like, "Oh, poor you, you must be taken care of"... even if you have to be a little sneaky to do it. After all, what are friends for? :^)