Best Option still a Pi (Score 1)

I think your best option is still the Raspberry Pi. You could easily assemble the Pi computer in a few minutes in the class room and students may learn valuable information about computers. I know several middle school kids who have very powerful gaming computers but use the Pi because of the multitude of things you can do with them. You could introduce this as a class project for all who want participate and see if there is a way to have those with more money contribute extra to cover those with not enough - or have a fund raising event like a 5k run or something.