I actually had a very different experience. In my elementary school, in a suburb of Kansas City, I was never taught that Christopher Columbus was anything other than a hero. We learned that many English and Spanish colonists treated the Native Americans badly, but that was it. We learned that slavery was awful, but we weren't taught the specifics. Until middle school, I thought that Christopher Columbus was entirely well-intentioned, and never meant to cause so much violence.
After I moved to South Florida, I was given a very different, much more explicit lesson on slavery and Columbus. But the modern difficulties of Black people were usually just mentioned in a hushed tone, or during some kind of open discussion.
Hmm... each student in my school had to write an essay on the ways MLK and the Civil Rights Movement changed America, and the winners read the speeches on MLK Jr. Day when a BLVD that was renamed after him, in a black church no less.
OH, and when I was in 2nd grade, in our class, we were told all the presidents and sang them in a song. On that day, I remember our teacher bringing a picture of MLK Jr, and talking to us about how he was a great man who won the Nobel Peace Prize for helping end segregation. I remember clearly our teacher (named Mrs. Rice) saying 'he is as important as any president we've had, and should be talked about.' In 3rd grade, we had to read a book about a young black boy living in pre-abolition South.
This was in Kentucky in the mid-90's. I think there was a strong push in the state to get to kids early to teach them why racism was wrong. A lot of pro-environmental stuff, too. Missouri, while a fellow Union border state, might not have seen this as a problem worth addressing so directly. So, I know that my anecdotes are not indicative of the reality for all American students.