Great idea ! But you are in front of lot of troubles :

* class are copyrighted to the teacher (or the school) in many crountries, you might have some law problems. * students are generally poor => free to use * students are genereally technology addict and know how to configure ads blockers. * be sponsored by universities

Ideas : * allow teacher to subscribe, and have a different interface to answer students' questions (students may discuss without teacher know, of course) * mobile app to study in transport * allow teacher to add optional exercice with correction (you do not want to be another online school and allows teacher grades their students)