Communism itself hasn't commited any atrocity. But some implementations of it have been attempted, resulting in authoritarian regimes that did commit atrocities. I'm not against teaching about those, so next time we try we can learn from our mistakes. But of course, communism = bad is not the way to do it, and communism = atrocities isn't either.

The Holodomor, the so-called "Cultural Revolution", and the Killing Fields immediately come to mind.

Rage Against the Machine had a song praising a Communist who got a lot of indigenous Peruvian blood on his hands.