The Wall Street Journal looked at the freshman class at 10 highly selective colleges (i.e. Ivy League), counted them up, and ranked the high schools — private day and boarding, public, and magnet — by the number of kids enrolled. While you would expect elite boarding schools like Andover and Philips Exeter to do well, Newton South High School (NSHS) and Newton North High School (NNHS) both made the list sending in 23 or 6% of its class and 25 or 4% of its class respectively.
The pdf is here.