I am pleased to report that I am writing this post via Raspberry Pi. It was dead simple to buy, set up, and start using.
At the school education level, Raspbian OS distribution comes with Scratch. Two more languages, Haskell and BCPL, highly promising for use in mathematics education at advanced level, appear to be easily available. From Martin Richards:
BCPL, an interpretive implementation of the BCPL language and system, including many demonstration programs. Click on BCPL.html to obtain a copy of the current version. This version can be installed easily on most machines running Linux, Windows and MAC OSX. In particular, it is easy to install this version on the Raspberry Pi machine. See the Young Person’s Guide to BCPL Programming on the Raspberry Pi (bcpl4raspi.pdf) for details.