You've solved some of the hardest problems already - getting the initial traffic, a decent product, initial user base, and marketing. You need to work on profitability and market size.

I agree that you should keep the free exercises to keep traffic flowing to your site. Don't focus too heavily on these, though.

Without knowing much about your market or product:

- Estimate the size of your market. A back of the envelope exercise should help you estimate how many schools are in the Netherlands, how many classrooms at each school, how many students. That's your current market size. How can you expand that? Can you extend to universities, or to other countries in Europe? Increasing your market size gives you more potential customers.

- You need to extract more money out of each user. $5/user/year requires a huge amount of users to generate even basic profit. Can you offer a premium subscription, for $15 per month? $20 per month? Compare this to the cost of private tutoring for parents. Put high value material in the premium subscription. Offer a 7-day free trial of premium.

I think you've got a good opportunity, you need to monetize it.