A potential idea would be to have students do there assignment online, and have a "playback" button to show the student working. A crude cheating measurement (or genius measurement) would be the time between answer submission and the time to completion.
From my friend's point of view, your service will not be considered without cheat-protection so I don't think it should be appropriate as a premium feature. I may be wrong, but my take on freemium is to allow the user to recognize the value when under the free plan and the use of premium to make things faster, larger, or better. In that vein, I would suggest automated grading for absolute or relatively absolute answers as a premium feature (i.e. integral x = (x^2)/2 or a single US founding founder would be part of a set {Thomas Jefferson, James Adams, ... }). That way, grading is completely automated. I'm sure teachers will pay for such a feature.