The Tulsa Race Massacre beats it by a few months, May 31st to June 1st. It leveled around 35 blocks of a wealthy black community, partly using firebombs dropped from planes, which was known up until that point as "Black Wall Street". A 2001 commission put the death count somewhere between 150-300 deaths, and noted that the city aided in the massacre.
They actually did teach me about that stuff in school. It was in the textbooks and everything. I don't believe the Battle of Blair Mountain was specifically mentioned, but there was plenty said about slavery, organized labor, civil rights, and so forth. I interpreted it as a celebration of our forefathers' victories over their oppressors, and a cautionary tale of what oppression looks like, so that we will never allow it to be repeated.
And now, here we are, allowing it to be repeated.
The content of educational materials is not the problem. Not the materials I was exposed to, at least. The problem I see is that most people don't think too hard about what happened and don't try to imagine themselves in the shoes of the oppressed. Even when I press people to think this through, they stubbornly refuse, often saying flat-out that they don't care. I doubt any teacher or textbook can fix that.