While academic material learned in school may or may not be useful once a student reaches adulthood, the same skills that create a superior student also lead to success in the adult world. Schools provide an age-appropriate place to learn and practice these life skills while receiving feedback on performance.
A student's report card provides more than a measure of the facts she has remembered. Report cards and grades also provide an indication of how much self-discipline and delayed gratification a student is capable of. The action-consequence nature of homework and test results provide an indication of the level of responsibility and self-direction a student possesses, along with the opportunity to learn how to take responsibility for change when a mistake is made. Dealing with school rules, other students and a variety of teachers all provide students an opportunity to increase their social and emotional skill levels.
High school provides a place to learn about the world, which both encourages and satisfies a student's curiosity, while simultaneously teaching students the research and study skills they need to continue their learning as adults.
All these are character traits that must be developed if a student is to become a successful adult. At high school, students have the opportunity to attempt using all these skills repeatedly, getting feedback in the form of grades and teacher comments. By the time they become adults, they should have fully developed the internal character traits needed to successfully take charge of their futures.