If you get into Vietnam at all, Tom Paxton's "Lyndon Johnson Told The Nation" is a good example of topical songwriting that dealt with fairly specific issues of the day. Country Joe McDonald's "Feel Like I'm Fixin' To Die Rag" is also a good one, and funny (helps keep the kids' interest, you know).

For the Great Depression, you might consider some of Woody Guthrie's songs -- "So Long, It's Been Good To Know You" is a good one, or "Do Re Mi".

The American Civil War produced some good examples as well -- "Dixie" and "Battle Hymn of the Republic" are a couple of pretty obvious choices.