I am not in Korea anymore, but one of the things that annoyed me was the fact that everybody got put into the same class levels as everyone else. In English class, that meant students that had lived abroad and could speak English fluently got stuck in classes with people that could barely say "Good morning!" Initially, there was some ability to overcome this by having after school classes to target the curriculum at a higher level, but before I left, even that ability was limited. One could have after school classes, but they couldn’t be called "advanced."
Thankfully, it seems that some changes are being made. Not in English, but some math and science students will have some opportunity to be taught something at a level and pace that is more appropriate.