Student usernames must be unique within the whole district account. If you run into a problem with a username that's already in use, consider using the student’s first name plus their last initial or some other way to identify that individual student.
You can also consult with your school or district administrator to confirm whether your system should be using a specific username/password protocol. For help on where to find your account admin(s), see Where Can I Find the Admins on My Account?