So basically while we know from chapter 43 that Kyou-chan's insecurities were there back in elementary school he was still capable of actually talking to people (though I'd guess he never got particularly close to anybody,) then the one-two punch of puberty and middle school came along and he just couldn't keep it together anymore and retreated into his shell, then painted said shell over with a thin layer of chuuni edgelord, a process that completed on or around that forest school trip.