First, click on the Manage Class button on the left side toolbar. Navigate to the class section where the student should be moved TO. Click on the "Add Multiple Students" button:
Next, click on "Search Existing Students" option
Next you'll be prompted to search for the student. You can search by student's name/email. Once you do that, you'll see the students matching the search term you have entered
Select and click on all the students that you need to add to your class. Next click on the "Yes, Add to Class" button.
You will receive a message stating that existing students have been added to your class.