My 12 year old son has been through several variations of the kitbag since diagnosis 2 + years ago Since he transferred to secondary school we have settled on using pencil cases.
They are not too embarrassing to be seen with as all school kids use them. They can be bought in a whole heap of colours/designs. All your mates know what it is if they need to get it for you. The thing to remember is that it needs to be roomy enough to fit everything in plus a few spares. It's usually the depth of the bag that causes problems. Really, I suppose it's like a make-up bag but for a boy!
Recently we have come across a fab bag (well, I think it's fab - son thinks it's humiliating!) called a Spibelt (http://www.spibelt.com). It's basically a very small but very stretchy bumbag on a waist strap. Shipping from the USA is hefty so we had ours forwarded by a relative in the USA.
It fits closely and is barely visible under a t-shirt/sweatshirt (my son believes everyone a)can see it and b)is staring at it!). It means I can let him go out and about with his mates on his own because he has his meter/glucose/snack etc physically attached to him and is less likely to lose them than when he takes his handheld kitbag and it is less cumbersome than a backpack (which he resists taking also - far too humiliating!)
Anyone else got any good ideas/products on the kit bag front?
