The main difference is that people are surrounded by far fewer people with whom they grew up but meet correspondingly more new, different and "interesting" people. And of course, many, perhaps most, people are away from home for the first extended period, which promotes homesickness, but also allows more opportunities for experimental adventures away from parental scrutiny.

For those who do not go to college though HS is often the first and last foray into adult privileges with little or no adult responsibility, as you and DK note. Most consequences are few, light and temporary, and people begin developing the skills and experience to realize a practically infinite panorama of potential, with plenty of time to regroup and redirect in the face of reversal and discovery. It is far easier to rebuild when the worst consequence of poor decisions/performance is summer school than when it means losing ones job and home, filing for bankruptcy and moving back in with parents increasingly dependents rather than providers. For many people it is the last time anything is possible and everything new, all within a safe familiar environment.