Is the student's email address not editable in Account Settings?
The School Admin must update a setting in the Organization Settings page under the Manager Tab to Allow editing of the Student Email. 
Once this is set to TRUE, the student can edit their email address in the Account Settings page.
Student's view of Account Settings page where they can edit their email address:
*If you are a school admin and wish to have the ability to login as the student to change their email address, please see this help article.