For that age group, if you are thinking of a one-semester or one-year class, I'd use VEX:
http://www.vexrobotics.com/vex-classroom-lab-kits.html.
1. Versatile mechanical platform. Broad hardware choices.
2. Multiple programming choices.
3. Fully integrated classroom and after school program (you may not think they want an after-school competition program, but there is no reason to paint yourself into a corner)
4. It all works together.
5. Fully reusable.
6. Multiple curricula choices, including one that is free with the kit.
7. For this age group, VEX is a better choice than LEGO.
8. Less expensive that other advanced kits (ones based on metal components and not snap-together bricks) with similar features.