If it’s OK to have some adult create a bunch of users you could do something like
gmailusername+fake-username1@gmail.comgmailusername+fake-username2@gmail.comgmailusername+fake-username3@gmail.com
to create a class full of kids “owned” by one teacher/adult/responsible person. Would this work for you (I just verified that I was able to add the +whatever to my gmail address on my own forum to create multiple users with a single gmail address).
You need some way to verify that they are people and not spammers, right? How are you planning to do that? (Maybe knowing that will help come up with a way to solve the problem elegantly).