This seems to be priced incredibly aggressively. $20 for ten hours of use - per month - is the cheapest plan. Someone using this full time would be forced to buy the $100/month plan for 100 hours, since the middle tier, at 30 hours per week, would not be enough for them. For a collaborative whiteboard? Essentially, "Draw Something" but retooled for education?
It looks nice. Can't you create the whiteboards so that the users don't need to login (except the creator of the whiteboard)? Maybe just creating a url that is a guid.
We could create a guest account but usually the tutor knows the student and will give more than one lessons. We also simplified the onboarding process for the student to a minimum number of fields.