Oh God, I almost forgot "Seasons in the Sun" (the Terry Jacks version). That song actually qualifies as outright depressing, rather than sad.

And also from that time frame (1973-1974), how about "Billy, Don't Be a Hero"? Man, I was like a sophomore in high school if I remember right, because the Vietnam War was still going on. A very traumatic song for a kid almost old enough to get drafted!