Typical situation: Physical education joined boys and girls from two classes:

One teacher has boys from two classes, another teacher has girls from two classes:

Use the button Joint classes for this:

If the school has many boys, they have to create two groups of boys. So we have 3 teachers, one for boys in 5A, one for boys in 5B and one teacher that takes girls from 5A and 5B:

Be careful: The lessons dialog in the above picture shows only the rows for lessons in 5A. So the row for one lesson for boys in 5B is not on this picture.