you've already admitted to doing freelancing while keeping the project going. You could of kept doing that. Theres companies that will pay for sponsorships if you have a 1000 dedicated teachers using your application. You said teachers pay for supplies, well you could make affiliate money. You could figure out how to get deeper discounts on bulk supplies teacher's are regularly buying and take a cut. You could ask for donations.
Once you have a 1000 people hooked on it, I have no doubts that you could think of a premium feature that opens new features to these users that they'd be willing to pay for. Again its about getting those users and showing them the benefits first.
It's an opportunity cost issue. The choice isn't go big or go home. The choice is work on this some more or [insert-new-project]. Which is more effective: to plan, host, and field support requests for a product that has no monetization strategy? Or to roll the lessons learned into a new project instead? The fact that there might be some value in 1000 users down the road doesn't mitigate the fact that he's learned a bunch of lessons that will inform his next product from the beginning.
Once you have a 1000 people hooked on it, I have no doubts that you could think of a premium feature that opens new features to these users that they'd be willing to pay for. Again its about getting those users and showing them the benefits first.