"Where is school number nine " without the article "a" is a better translation for "де школа номер дев'ять?". This should be accepted as a default answer. I am not sure if the given default answer with the indefinite article "a" should be accepted at all. Probably not. "Школа номер дев'ять"="school number nine" is definite and therefore does not go well with the indefinite article "a"