- Don't sell to students, sell to parents (apart from the schools of course). They are more motivated to than children to pursue good grades and they have money.
- Charge more. If you lose users, so what? You've only lost the ones that you can't make money from in the first place. (although I see that you've raised the price already).
- Your wife doesn't sound committed. No selling the thing (migrate to a new domain now to get rid of the name association, what's the problem?), no charging more out of fear, fear of losing control. If it's a hobby for her, and you want it to be more, you have a problem. Solve this first.
- Find subject experts on a profit-sharing basis if you don't want to invest any money (although seriously at your age and your household makeup, 5000 euros should not be a major problem and it would get you sufficient content for 5 different subjects).
- If you will work 4 days after your baby is born, you'll have even less time than you have now to work on this! This one day will be full-time babysitting unless you take her to day care, which I presume you'll only do for 3 days a week!
- The way your website is structured, it doesn't seem to be easy to expand into different subjects.
- The 'original' website (with the free content) looks like a 1997 Geocities page. It also doesn't make many attempts at upselling, apart from some small links to the premium version.
- Have you tried calling schools, or the associations your wife is a member of? If she uses it, ask her colleagues why they don't. Then fix those issues.