You won’t be able to add a student who mistakenly created a teacher account. To know if the account your student is using is a teacher account, proceed to their profile. A student profile should look like this:
If your student created a teacher account, before they can create a student account with their email address, they will first need to delete their teacher account. Proceed to the profile and scroll down to the bottom of the page to find the “Delete my Account” link:
Confirm your action to delete the account. Your student will then be able to create a new account using their email and student code.