For CAD you can run Solvespace on a chromebook. I'm not sure I want to promote it for use below high school, as the geometry failures and their workarounds are probably too icky for younger users. I want more polish for kids.
I think a set of problems where you intentionally create failures and then work through solving them would be quite interesting for anyone learning geometry, beyond a certain point. Understanding exactly why the failures are failures, what about the geometry or the computation makes them fail, would help build an intuition for geometry and topology itself.
Solvespace is hardly polished enough for general use, though. I can use it, but I constantly struggle against the way it handles sketches.