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.
The OP is in Australia like me and while there are lot of parents that can’t afford to pay, there are lots of that could. The key here is to build your business around the customers that can pay, not those that happen to use it.
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.
