Hi Sherry
Your user has logged in so his account is confirmed (that is when a new user signs up but doesn't complete the confirmation process)
When you are adding the teacher check the 'Enrolment duration' setting (which is displayed between the role you are assigning and the boxes displaying the existing and potential users) to ensure it does not say 1 day.
Also check Administration ▶ Server ▶ Cleanup to ensure the admin setting isn't "cleaning up".
I hope this helps
Cheers
Teresa