Choosing Teacher when you sign up creates two accounts on one login: the teacher account you asked for, and a student account beside it. You never register twice and you never have a second password.
Switch between them with the circular arrow in the profile menu in the header. The first time you switch from student to teacher the platform asks for confirmation; after that it just switches.
Opening a lesson as a teacher switches you into the student account automatically — that is where lessons are done.
What the two accounts share: the login itself — email, password, name, avatar, interface language, notification settings.
What belongs to one account only:
Switching signs out your other sessions. If you are also logged in on your phone or in another browser, that session ends and has to log in again.