That is a very good score for an 8th grader, and better than average for many good public high schools. If you look at the CTY data in 2014 about 1000 8th graders took the SAT and a 620 Math is in the 53rd %tile. Now remember the CTY pre selects the top 5% on various standardized tests here is a link for their data.
http://cty.jhu.edu/talent/docs/test%20data%20satact/2014SAT.pdf
http://cty.jhu.edu/talent/after/about/index.html#SAT_ACT_Data
The Dukes program tests more kids, sample size was 15,000 but they only had the 7th grade test stats, but I think they pre screen less as the %tiles were skewed greatly from the CTY data and probably represents the general population better.