If you are having problems adding a class to your school structure, it could be because you already have pupils added to the year group level into which you want to add the class. To change that 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')