) Read every single business-related post at patio11's blog [1]. That is great advice for any startup (due to the quality of the content), but for a startup selling to educators? Unmissable. In case you are unaware, patio11 is extremely popular on HN (for varied and valid reasons), and he runs a website which sells bingo cards to teachers.
) Agreed re: adverts - as a user, I would be annoyed that I am paying for your site and still seeing ads.
) Try outsourcing the creation of new material - the more subjects you cover, the better. There are many freelance where you can find teachers in various subjects, and your wife can act as "quality control".
) Use Adwords to bring more people to your site
) If you are specifically targeting Dutch schools, maybe creating a Hyves app will help? (Unless that is your employer :D ) [1] http://www.kalzumeus.com/
) Perhaps try to attend some startup groups in the Netherlands? I live in Amsterdam, I should try and find one too... maar mijn Nederlands is niet geod.