"士官的二等室" can be translated as "Second-Class Quarters for Non-Commissioned Officers" or more concisely as "NCO Second-Class Quarters" (where NCO stands for Non-Commissioned Officer).
Here, “二等室” implies a certain grade or level of accommodation, and “士官” refers to non-commissioned officers in the military. The translation captures the essence of the original phrase by indicating that these quarters are specifically for non-commissioned officers of a certain (second-class) grade.