In order to transfer data from one class to another, you must first transfer those students that belong to both if you haven't already. The details explained here
You can either copy a single column, a selection or a whole tab.
To copy a column: Tap&hold on the column's header / Copy / To another class
To copy a selection of columns: Tools icon (top bar) / Select your columns and Copy / To another class
To copy a whole tab: Tap&hold on a tab / Copy / To another class