One Strap BackPacks for Tween Girls and Boys, in assorted colors.
Pictured school bags available through Unbeatable Sales, Inc.

Sling backpacks are a popular new style, and make a lot of sense for tweens and teens, being that one strap carrying is more common, than putting a backpack over both shoulders in traditional carrying mode. There are many colorful styles available, for both boys and girls, as pictured above (images link to pricing details and availability).