I remember my 5th grade teacher back in 1977 stressed that the Holocaust must not happen again and she did mention that “there was one going on now” (in 1977) and told us about the Khmer Rouge.
I think when it was happening most people didn’t know what was going on; Western journalists that wanted to play “Hanoi Jane” literally disappeared (the Khmer Rouge viewed anything “Western”, including glasses and literacy, as evil that polluted their pure farmers’ utopia - they didn’t need some Western Benedict Arnold to sit on an AA gun for them). I wasn’t even in grammar school when it happened, but they kept a tight lid on their madness.