Find the student in the wrong class by clicking on 'English Every Day' in the menu on the left, then on 'Classes'. Then select the student's current class.
Click on the name of the student. It'll take you to their page with the English Every Day statistics and the class they're in.
Right under the student's name you'll see the options of removing the student from the class, of changing their class or changing their password. Click on the second option. You'll see a window with all your classes. Choose the right one for the student and click on 'close'.