We (principal and I) have agreed that the legal aspects are too complicated to evaluate for such a small student project, so we will simply return all rewards that are made by students during the course to the owners of Steemit.
Ideally, every student should of course keep their rewards, but I will try to shape the tasks in a way that don't make this a competition between students for rewards. Outside of the classroom they can use Steemit as much as they want of course :)