Unfortunately, it's not possible to turn a teacher account into a student account. However, If you have some students with teacher accounts, tell them to go to https://edpuzzle.com/logout and then sign up as a student. Edpuzzle allows you to have a teacher and a student account with the same username/email. This way, you will not have any problem creating the new student account.
If a student wants to delete that teacher account, s/he will have to contact us directly, requesting for us to delete his/her teacher account using the same email s/he used to register.