Working with divided lessons

Creating a divided lesson:
1. Select subject and class by clicking on the corresponding cell. Type 3 as count of lessons per week. Then press right mouse button on the created lesson and select the corresponding division:

2. Now you can select the lesson teachers:

Notes
If you want only one group to have this lesson, then simply unclick the corresponding checkbox:

If you want to change the division or clear the division, click right mouse button: