If you have accidentally removed a student from your school you can drag them back into the school:
- Click manage students
- At the bottom of your list of classes there is removed students - click the triangle next to this
- If you do need to choose more than one student, then click 'select multiple students' under one of the main classes on your roster
- Click on the removed student and drag them back into their class
If you clicked 'clear all' next to the removed students i'm afraid the accounts will be unrecoverable.