He's wrong.
The stats are a bit slippery, but the best study I'm aware of put the national graduation rate at 71% (1988).
http://www.manhattan-institute.org/html/cr_baeo.htm
To get into the mid 50s you either have to look at rate for for African-American and Latino students nationally, or the overal rate for a state such as Georgia.
http://www.manhattan-institute.org/html/cr_baeo_t2.htm
According to most recent data, which should have been available to Kerry staffers...
http://nces.ed.gov/pubs2006/2006601.pdf
The actual stats are 73.9% of freshmen graduate....
"The [averaged freshman graduation] 2002-03 national rate is 73.9 percent.
In summary, close to three quarters of freshmen nationwide graduated from high school on time in these 2 years (tables 1 and 3)."