OK, open mouth, insert foot. I'm going to guess that that cover was compression molded, probably of phenolic. The compression stuff is more brittle than the injection molded, and maybe the cure portion of the mold cycle or temp or both were a little off. So along comes Mr. Maintenance with a vacuum or a polisher, and when he's well away from the receptacle, he yanks out the cord. Do that with the lower outlet, and you tend to break the bottom of the ground pin hole out (like here). Do it on the top outlet, and, CRACK! Off comes half the cover.
Isn't it embarassing for the school to have something like this in a CODE classroom?
I hate to think how often I have seen receptacles like this. Wouldn't you think the liability alone would move property owners to have something like this fixed?
Just by the by, I find it interesting that this unit uses the T-slot contact terminal assembly on the hot side as well. I suspect you could slam a 240v plug through that plastic if you tried. Not that anyone does things like that...