I have never noticed that before, but it sounds quite logical to me considering these points.

1. The character can look to the outside world giving more freedom to new plot lines.

2. Less background students make it easier to draw focus on the important characters, especially during class hours, when the seats must be taken. 

3. Maybe just personal preference, but I think it is nicer to have pictures with some contrast (window-landscape on one side, tables on the other), than just a dull matrix of tables.