The students who tended to do best in the DCs' high school were those who participated in extra curriculars. Studies indicate that this is a general phenomenon. SheGot's HT is correct.
The phrase 'if you want to get something done, ask a busy person' applies. The busier > the happier > the more productive the student.
One of the students in DD1's year who got As in all subjects all through his high school career and graduated at the top of the class did three sports every year, each at varsity level. He was captain of all three of his sports in his senior year. He got into the Air Force Academy.
Introverts don't all spend their time with with their heads in books. Some like making earrings and watching crappy tv on their own. DD1 is most definitely an introvert and needed time alone in her room with the cat to recharge. She also found her own space with her music on and headphones in, even in a crowd.
Indeed, there is complete acceptance that all brains are not the same in what can only loosely be called "the US system". Crucially, there is acceptance too that all brains do not remain static. Hence mixed ability classes in elementary school, where the basics are presumed to be within the reach of everybody, and hence also different tracks and levels (sometimes designated 'advanced', 'college prep' and 'general') within tracks in high school. The lack of setting in elementary schools means that students who despite appearances at age 6 may have ability or are willing to do the work can get where they want to be in the end because they have not fallen so far behind that they can never catch up. DD3's maths teacher was out a lot with health problems for her last two years of middle school and DD's class had a succession of subs. The result was confusion on DD3's part. She placed in a college prep level in her HS maths placement test, but did two years of summer school maths as well as her regular two full years and therefore advanced four years of maths levels in the space of two. She is now on an advanced maths track.
One key element in many US states is that the majority of students must take English and maths for most of high school, with many districts and states requiring a certain number of semesters of science, art, social sciences and a mfl too. Schedules and class makeup can be flexible because many high schools are large. Students may or may not have students all their own age in class with them. DD4 has bearded 18 year olds in her computer apps and art classes -- these classes are a graduation requirement and some students leave it to their last year. DD3 had older and younger students in many of her history and maths classes.
There is no finishing post in US schooling such as the Irish Leaving Cert or GCSEs, A levels, etc. Instead, students aim for a good grade point average (consisting of the grade for every homework assignment and test for your four years of HS). After four years of high school you can apply to university if you wish and chances are you will get in somewhere. Universities sort applicants by nationally administered exams that measure aptitude and competence and also mastery of certain subjects, as well as evaluation of the quality of students' HS courses and grades.