Earlier quoted context omitted.
James I think you are focusing on the wrong market. Your potential paying customers are not students, but their parents who have the money. One idea might be to allow parents to track what their little darlings are doing on the site. Another is to provide regular feedback (email, newsletter) to the parents about how their children are progressing.
Not sure how things are where OP is located, but some parents may not be able to afford something like this depending on their income. If this works well integrated with school activity maybe it could be payable through that?
One other idea might to try and partner with companies that are trying to lock in young people. The obvious one is banks which are always trying to target this age demographics. You could offer, for example, premium notes to those students that had an account with the sponsoring bank.