It's a linked dupligroup.
A Dupligroup means that the plane (it could as well be an Empty) generate a group instance at his position. You can verify that in Object/Duplication. It allows to manipulate easily the whole desk, despite it is composed of several different objects ; and it's memory efficient (in Blender's mind, there is only ONE desk).
Link (linked library)
In the outliner, you can see the list of the different groups available, by selecting "Groups" instead of "All Scenes". As you may notice, the "file with an arrow" next to the name of the group indicate it is a Link : it refers to a group that is in another file (in assets/desks.blend).
You can open desks.blend, and try modifiy something on the desk (ie, change one of the materials). Save desks.blend, then open (or re-open) classroom.blend : all the desks have been modified.
There is many advantages in this workflow. It allow easy reuse of assets, it's flexible, it allow one artist to work on the assets and an other on the scene...
It's also efficient because the group is loaded in memory only one time. And the size of each blend file is kept small, this is easier to work with many small files than with a big one.
In myAsset.blend, create your asset as you would usually, with different objects, modifiers, etc. Once finished, select all your objects and press Crtl+G to add them to a new group. Give the group a meaningful name (in the outliner, or in Object/Group). Put your asset at the center of the scene (0,0,0) because that will be his Origin.
In myScene.blend, click File/Link, navigate to myAsset.blend/Groups/myGroup. Now you have an instance of the group in your scene, you can manipulate it like any object. To add another instance, just press Shift+A /Group Instance/myGroup.