Do students need an e-mail address?
I am about to start using duolingo with my middle school students. Can they register if they don't have an e-mail address?
Yes, you do. You could just type in a fake email but if someone forgets their password then there's pretty much nothing you can do about it. They should create an email because it is quick, easy, and you have a way to communicate with your studens.
It might also be a possibility to use a parent email. I would think that the average middle school student likely already has one they use to register for other websites like facebook, instagram, etc.