If not a school, some kind of sexual training programme or series of lessons? It's a common trope in fiction, appearing in Firefly, Game of Thrones and Metal Gear Solid. Is it entirely fictive and fanciful, or is there some truth to this trope? TvTropes only lists a few unsupported claims of such institutions.
edit: more information
and yes I know this technically breaks the 20 year rule but given the fact that the practice was started under Kim Il Sung and continued under Kim Jung Il I figured it would be applicable.