Typical situation: Physical education, 3 groups in 2 classes

We have girls joined from 2 classes, however boys have separate lesson in each grade.

Solution:
You have to input 3 lessons.

1. Boys in 5A:

2. Boys in 5B:

3. Girls joined in 5A and 5B:

4. Result will look like:

Sample file can be downloaded from here.

Note: for each lesson we have specified that it can be in either of 3 available Gym rooms.