남고덕뿐 is not an easily understood phrase. It looks like the author coined it to suggest the idea of 남고 (boy's school) + 덕(probably for 덕후 which means the Japanese-originated 'otaku' for geeky enthusiasts) + 뿐 (nothing/no one else). So it might mean a boy's school where there are only eccentric ones. But it's just a guess and I could be wrong.