Overriding a students username is not working correctly

If I override the username of a student and set it to their personal email. They will get logged in, but later be unable to log in because their username has changed to their partial school email instead of their personal address. For instance, if their school email is username000@school.org, it will change the username to username000. But it should've been studentname@gmail.com, which is what I designated.