I would start with Lego Mindstorms. They are expensive, though, but will be well-used and a great educational tool. Many communities (general & HS'ing) have 4H Robotics clubs or First Lego League teams.
Northwestern University's Center for Talent Development has online courses in Mindstorms called "Robotic Discovery":
======== COURSE I: In this hands-on, self-paced robotics course, students learn engineering, building and programming, using the LEGO NXT Robotics Design System. Participants create their own autonomous robot, applying their new engineering knowledge through interactive exercises in math, science and physics.
* Parents are encouraged to assist their students with manipulation of materials, as needed.
* To ensure communication with the instructor, CTD suggests that an inexpensive webcam and/or a digital camera also be purchased.
* To ensure student safety, parents must be present during webcam communications.
* The LEGO Mindstorms Education Base Kit (#9797) and the LEGO Mindstorms Education Software (#0077)NXT Kit must be purchased independently by families. The kit and software are available at the LEGO Web site at http://shop.lego.com/product.asp?p=8 527 or at online and retail stores. Students choosing to use the retail kit must be prepared to make some adaptations to the models built in the course.
COURSE II: Expand on the engineering, building and programming skills developed in the Robotics Discovery I course. Using the advanced features of the LEGO NXT Robotics Design System, students design and build a unique autonomous robot, complete with light, sound and touch sensors.