Upvote:3
I couldn't find any schools aside from the one you have listed. Generally speaking I would always start from the sports associations like Korean Parachute Association but their site is all in Korean and rather poorly done too.
There is also a Sky Divers' forum called Drop Zone which has a section dedicated to South Korea. It might provide some information but I didn't feel like registering to get at the content (sorry).
There also is a list of Dropzones across the world and prices to access them as well as clubs servicing them but nothing in South Korea.