You cannot please everyone (be ok with firing some customers). This will affect what and how you develop your product. Additionally, keep it simple and make it easy for teachers to get started, deciding in advance your monetization strategy. Nothing will cause a greater outcry than putting previously free tools behind a paywall because you decided one day it was time to make money.

Bonus: research funding opportunities on schools' behalf so they have an easy way to pay for your product. Are there grants available that your product would qualify under? Could Title 1 funds pay for your product? Etc