There are already Arduinos and regular PCs (of which you will find far more documentation on... especially the latter) for that. Why make another closed proprietary system just for teaching? All it will do is make those educated (indoctrinated?) by it think this type of environment should be the norm and encouraged, when there are in fact plenty of better ones.
Even a C64 or a ZX Spectrum would be a better choice than an RPi for teaching.
This is the strangest opinion I've seen all day. My teen programming years were on a c64 in the late 80s, and in retrospect that was a really tough learning experience (and when I got to college and had access to 'anything else' I never looked back).
Also how was the c64 not a 'closed proprietary system'?
It's been completely reverse-engineered, emulated, and documented, a lot more than can be said of the SoC in the RPi. Maybe in 30 years the same will happen to the latter, but I'm not too optimistic about that.
C64: http://www.classiccmp.org/cini/pdf/Commodore/C64%20Programme...
C128: http://www.pagetable.com/docs/Commodore%20128%20Programmer's...
Because that's still not equal to an open-source design where the licensing eliminates such risks.