[brudgers]

Have you considered tying your project in with the Racket folks. Matthies Felleisen (The Little Schemer) and that community have a considerable history in designing teaching languages, and Racket has substantial tools for developing languages tailored for learning, e.g. friendly error messages. It also has done a lot of heavy lifting for graphics, webserving and html.

It looks like Isla might slot in to the front end of the spectrum before BSL (Beginning Student Language).

Their middle school resources are here: http://racket-lang.org/learning.html

The technical reports linked from the page might also provide some background in the design of teaching languages.