...But everyone is challenged in high school.
Really you think everyone iin H.S. is challeged? What gives you that idea? H.S. performance, grades and challenge are three different things that don't necessary correlate. My DS goes to a top rated H.S. and there are many kids who are not really challenged for one reason or another. This year my son was just dropped from the "gifted" class because of grades. (It's complex looks like we have a 2E situation.) He is certainly not going to be challenged this year. I know of kids who don't like the pressure and intensity of the AP classes and stress for top grades and give up trying. It's complicated.
