Shouldn't surprise you that much, given the national average is around 70%.
Zip back 25 years to the late '70's... did we have dropouts then? You betcha. Zip back 25 more years. Dropouts then? You betcha. Difference is back then there were factory jobs to go to. No more.
I'm not so old I can't remember numerous faces that started high school with me that didn't share the stage on the final night of senior year. Back then it was the student's responsibility to get work done or face the consequences, and if the student couldn't hack it, out they went. But just for jollies... ask your grandparents how many of them graduated, or for that matter, how many of their friends graduated high school. Where I live it was a really, really big deal for people during my grandparents' time.
I guess I'm saying the view of graduation rate has changed (that is, the public perception of its importance), but if anything we graduate a higher percentage of students now than we ever did... and I'm not sure it is a good thing (the way we have accomplished it).