I was genuinely surprised to learn this in high school (for me, 2017-2021 or so) - not because it's not a notable event in US history but because I was surprised my history teacher was allowed to teach it.
High school teachers are not given free reign to teach whatever they want. Even college professors are currently being threatened. See all of the rules being written about "CRT" being disallowed in curricula (not that the authors of these rules even understand what CRT is) or the mandatory ideological surveys of college professors in Florida.
We (the US) are currently in a debate about banning the teaching of “critical race theory” in schools.
Many history text textbooks downplay slavery to such an extent that most high-school graduates think the Civil War was about “states rights” when it was really about several states committing treason in defense of slavery.
The genocide and ongoing mistreatment of Native Peoples is barely covered.
In some states, teachers are required to sign documents saying they don’t support the BDS (boycott, divest, sanction) movement against Israel.
Telling teachers what they are allowed to teach doesn't really go against free speech. I can talk about how I think the earth is flat for as long as I want, but if I start teaching that to students I'm going to get fired (hopefully).
Provide evidence. In Washington State the genocide of Native Americans and Black slavery was the topic from elementary through high school. I can't remember a single year either subject wasn't explored, critiqued and analyzed.
1: Only 8% of students understand that slavery was the cause of the Civil War.
2: Here is a book that destroys the myth of the Alamo, and points out that the white people in Texas went to war twice to keep the institution of slavery.
3: There are lots results for problems with the way native history is taught in the US. I grabbed an interesting read as an example. As further evidence, until a few years ago, the Washington NFL team used a racist slur as its name