If you have accidentally removed a student from your school you can drag them back into the school:
1) Click Settings at the top
2) Click MANAGE in the Manage students box
3) At the bottom of your list of classes there is a folder called Students no longer in my school - click the triangle to show the list of students who have been removed
4) If you need to choose more than one student, then click 'Select multiple students' under one of the main classes
5) Click on the removed student and drag them back into their class
Note: Students will remain the Students no longer in my school folder for 6 months before their accounts are removed from the school. This is to allow them time to either join a family or a new school.