@Susan: I'm not an Arduino user, but I've used the under-the-hood AVR microcontroller in quite a bunch of projects. I 'm not sure about using the Arduino with educational purposes, as it has been thought to be programmed using an extra layer of software abstraction -- instead of using the C commands for accessing the AVR resources, you have the Arduino API.
Raspberry, BeagleBone, etc. are the ideal tool for learning OS, both Linux and Android. The students can play with the whole filesystem and it they blow it away, the problem can be fixed by just replacing the storage SD.