I had a website that I was selling advanced CBT on it, like PHP, C, cisco, perl training videos, and it was doing very good. I had to stop it because the items that I was selling were illegal, and this was the only reason. :)
But maybe the idea can appeal to the growing nontraditional markets: tutoring, distance education, self-education, corporate training, for-profit higher-ed (U of Phoenix, ITT, etc.).
Also: internet in the schools brings advertising at a level far beyond the controversies that affected ChannelOne/Edison in the 90s. Not all school computers have ad blockers, do they? So perhaps even an ad-supported model would work for a really lean educational web-app startup -- and there's little more lean than a single teen founder.
And you will know.
Just see if you can do it alone, if you can't, then try to ask one of your friends to do it with you.