If you've accidentally removed a student from your class, the easiest way to add them back is to use the 'add a student' function. 

Step 1: Go to My Classes and click into the class with the student you want to add back. Then select 'Manage students' on the left side menu.

Step 2: Click 'Add students'. Use the option 'Add an Individual Student'. Click 'Invite student'. They will automatically be re-added to your class.
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Note: Make sure you get the email address correct, otherwise you might invite a completely different student! Or no student at all! 

Alternatively, you can also share the four-letter class code with the student. They can enter the code on their subjects dashboard to join the class directly.