What microcontroller is best for education?

Rukie writes: "I'm looking into starting some sort of robotics class for my high school, which severely lacks any sort of technological classes. I am now wondering what microcontrollers are best for an educational environment. I definitely want something more advanced than the legos, but something that won't fly over people's heads. Are there cheap, scaleable microcontrollers for learning in a classroom or at home? I'm curious how the Slashdot community has hacked up toys to make their own robotics at minimal cost."