That is Yotsuba's room. It's one of the 5 rooms shown in the previous chapter, and its also not the only one that isn't a "classroom". I don't know if its a translation or author's error that "classroom" was mentioned before.
As for why she left and came back, clearly she thought she was not going to be chosen...she didn't even harbor the tiniest hope that she would be. In keeping with her past behavior.