Free access to your SaaS for friends and colleagues?

I have an education-focused project on hold right now. If I polish it and release it I'm going to make it free to teachers in the school district where I used to work. That's easy, because it's a simple email domain rule for granting free accounts.

Having some free accounts to seed activity and test out features is really helpful, but as others will say, if no one is willing to pay that's a really important piece of information.

What’s the harm? On the other hand feedback from a thousand free users is worth less than the feedback from one paid user.