If the student stated their school when registering, go to the Menu and select the Classes & Settings page.
Select 'Classless students' from the drop down list of classes (this should be at the very bottom of the list).
Find the student you want to move and click on the student's name. Use the 'Apply Action' drop down to 'Move Class'. You can now place the student in their correct class.
If you can't find the student in the Classless Students list, it's worth checking the Archived Students list as well to make sure they aren't there.
If the student registered without stating their school or selected the wrong school, then the student will need to go to their account settings and complete the 'Request Change of School' form. This form will be approved by an administrator and once approved the student will appear in 'Classless Students' for you to move to the appropriate class.