If you are having problems adding a class to your school structure, it could be because you already have pupils added to a particular year group. Classlist thinks this particular Year Group only has one class. To change your year group to a multiple class structure you need to:
1. Move all pupils out of the year group you wish to add a class. Try putting them in unallocated temporarily.
2. When the Year Group is empty of pupils, create a class and click on Add.
3. Shift pupils into the correct class.
Example of a school where you CAN and CANNOT add a class to a particular YEAR
CAN add a class
CANNOT add a class (you need to shift pupils out of 'Year', add a 'Class' and shift them into the new 'Class')