Although this question is one year old, I'll leave an answer for the sake of new students:
The verb "to be" is implied in это, that's why this kind of sentence always use это in Russian, otherwise the sentence would end up with no verb.
If I was in school and in the hallway are a lot of tables, would the teacher say that phrase, too? Can you say "чьи", too, although they don't really have an owner, but it's more important to which classroom they belong.?