I just can't believe that there are no other 4th graders in his position in such a huge school district. The funny thing is that by the time he gets to 9th grade, there should be a few other 9th graders taking calculus since there are a few of them doing that now. Apparently, a few kids manage to accelerate a year between middle school and high school, probably through summer programs like CTY. DS is not anywhere near a prodigy and he is really more of a verbal kid anyway so math is not his strongest area. Where are the other kids?!!
I don't know how big your district is, but for sake of comparison, I don't think there would be any 4th graders at all in my district (which serves probably 150,000) doing this. Also, I did not know of any 9th graders taking calculus in my extremely high-rated high school (this was, of course, many years ago and these days kids may advance more at young ages). So, I don't know, but your measuring stick could be off?