[murgindrag]

This seems like a neat idea. Not sure about paid, though, in its current incarnation. There's a ton of similar things unpaid. Why would I pay for this over Khan Academy's excellent SQL course, for example?

It seems like a piece of a business, not a business in itself. To be successful as a product or solution, there needs to be more around it -- some way to integrate it into contexts, have it used in schools, or in afterschool, or something.

Another key component would be user studies -- develop PCK about what supports and help students need.