Minix?
Stop sniggering at the back there. I wonder if you could run Minix on a Pi? It would have to be an old version, like v1.5, which I think was the last version with an ARM port, and not the redesigned v3.
Minix was designed expressly for the purposes of education by someone who really knew what they were doing and is small enough (~10kloc) to be comprehensible. It would also be an easy stepping stone to Linux for users of that bent.
Just a thought...
Biting the hand that feeds IT
