To sign up for Duolingo, only an email address is required. The reason we need that is in case the student wants practice reminders and password reset instructions. The email addresses can belong to a parent, teacher or language learner, so adults can choose to be in control of the account or even keep it anonymous if they reach that agreement with their student.
You can change the email associated with an account at any time in the account settings. Note that if you have an account that is restricted by age, you will not be able to edit your email address.
Please take the time to talk to your students about appropriate and responsible internet behavior and safety.