I want to gather a good audience in 1 month. What's the way to go in 2021?

Hi, I want to gather a good audience before I launch a startup of mine so that I can get people subscribing to my email newsletter asap. My main focus is teachers. I originally thought a Facebook group would do the trick, but I've no idea if it's hard to go semi-viral there (30k members is my goal). I'm willing to put out good content every day and answer community questions etc.

I'm a broke student whose max budget would be 100€. What's the best bang for my buck AND time in 2021? I highly appreciate any kind of advice.
Many many thanks.