I don't remember if there were even questions about it on the AP test. My APUSH teacher was great, but he didn't spend much time on it. We only had around half a week to cover Vietnam because everything starts to get scrunched up towards the end of the year. That meant Korea was limited to trivia questions (like you said).

You have to seek it out on your own during and after high school to find anything of substance. US sources are also not very good. They don't mention things like the labor strikes preceding the war. Nor do they acknowledge southern leadership was comprised of Japanese collaborators. These were the main reasons for the war, but the cause among English sources is "North Korea just did that. They spontaneously sent an army to kill everyone in the south. Nobody knows why because Kim Il Sung was a dictator."