I'd recommend broadening your perspective - stop thinking about how to find them online, where they are probably going to ignore messages as spam anyway, and start thinking about where they really are. Which is at schools. Think about how to approach them within that environment. Email the school or district administration, for example. Or local/regional organizations and associations.

When you do reach them, don't sell - ask to collaborate on what the product should be. Partner with them, don't treat them like targets.

This is true for all industries, by the way. The biggest error I see when people go looking for their users is to have blinders on and only look at where they exist online, instead of where they exist in real life.