[jpolitz]

Consider Bootstrap:

http://www.bootstrapworld.org/

Bootstrap 1 is computing tied to algebra concepts, targeted at the middle school age group (US grade 6-8). Students learn things like order of operations and what a function is while building up their own game. The curriculum has lots of activities that show students how to use code to generate images early on, which gives something immediately interesting and tangible to work with.

It runs in a stock browser (lots of students do Bootstrap on inexpensive Chromebook models), has a great support mailing list, and is used in schools across the US already. It also explicitly helps with math skills, which can be doubly useful in preparing for other STEM topics in the future.