I'd echo those who suggest "This Land is Your Land," with the caveat that it ought to be sung with ALL the verses, including the ones Woody warned Arlo not to let people forget--the verses that nobody included in the sanitized school songbooks.

Another thought-- how's about Buffy Sainte-Marie's song, "Now That the Buffalo's Gone" or Johnny Cash's song, "The Ballad of Ira Hayes." Not everyone knows them, but these songs capture the effect of colonialism on some of North America's indigenous people, and that's a big part of our nation's story.