How to specify lessons where students are joined from two classes?

To specify such lessons you can use button "Joined classes" in "Lesson" dialog.

After pressing this button a new window will show up and in particular rows you can choose Classes from which students are joined in this lesson.

In case you want to combine only some students of the class (for example only group "Boys") then you need to select field "Group" for each class.

There are 8 slots to quick selection. If you wish to join more classes, click button "More" and then select each class with group one by one (in order "A-B-C"). Confirm final selection with "OK" (D).