You can easily add an extra Base Class to any year. Base Classes usually have between 24-30 pupils. You should only add an extra Base Class to allow for an increase in pupil numbers.
Go to the Class Data Screen and then go to Actions
Click on 'Add Base Class' and then enter a single alphabet letter for the new Base Class
When Save is pressed, you will see the new Base Class as in the screenshot below.
Please Note: - A new Base Class should not be added unless there is an increase in pupil numbers. If you want to be able to lengthen a block (eg a four-way divide for 3 base classes) it is a new Dummy Class that should be added.