"Where is the classroom?"
I think クラス is class as in the intangible thing (e.g. the class of '98 or to teach a class) where 教室 is the physical room. You could ask where the class is (e.g. クラスはどこですか), but my prompt asks to translate classroom explicitly.
I felt the same way. Where i live we hardly ever refer to it as a class room, just the class
When asking a question in Japanese with "desu", the question word should attach to "desu".
[topic] wa [question word] desu ka?
教室はどこですか (kyoushitsu wa doko desu ka)
Where is the classroom?
If you use a different verb, then the question word can come at the beginning of the sentence.
どこに行きますか? (doko ni ikimasu ka)
Where will you go?