I'm willing to bet a majority of American citizens do not know much about our history.
That said, I disagree with Santayana's sentiment that those who forget history are condemned to repeat it. Humans are condemned to repeat history, regardless. The same themes keep recurring, history tends to go in cycles (modified for the era). Because until our essential natures change, humans remain humans regardless of the time period with the same concerns and aggressions. In my opinion, though, while history cycles, continued social progress (as I'd define it) serves to reduce the amplitude on those cycles, causing less extreme swings towards violence and hatred. So, in that sense, even if individuals don't have a good memory of their high school history, memory of history tends to get baked into the development of society.