Go to; "My Subjects" and click on your class.
Click on "Class groups."
Here you can create two class groups - in the drop down next to each child, select if they should be in Group 1 or Group 2 ( you can select up to 10 class groups.)
Now, when you run the Markbook you will see "All Students" click on the drop down list to choose 1 or 2 and click "Choose."
Note: You will have to select one group at a time to view the data.