One of the worst things I remember about high school is not the amount of homework I had, or the exams, essays, and various individual assignments. It was getting up at 5 am Monday through Friday. Ironically, during the time of my life when I needed the most sleep, during my teen years, was when I was getting up the earliest.
Many studies have proven that when kids are in high school, their minds, as well as their bodies, are still going through an important stage of development. In fact, our minds don't finish developing until we're 25. Teens typically need more sleep than their parents, which is why getting up at 6 am is often easier for someone out of high school than it is for a 15 year old. In other words, it's not because of laziness, but literally because teens' brains work in a different way.
Someone who's not well rested isn't at their prime when it comes to productivity, especially in Algebra class at 7 am. A lot of the time, I would come to school and my first class of the day would be study hall. Instead of using this time to do extra work, which is what I would have liked to do, I ended up accidentally falling asleep in a good number of those classes.
Of course, this could have been avoided by going to bed at 9 or 10 at night, but this isn't always possible for teens with a heavy workload, on top of other extracurricular activities, which might keep them up a good deal later. Not to mention social or familial obligations, as most high schoolers' lives don't consist entirely of academics, nor should we expect them to.
The American Academy of Pediatrics claims that school starts too early as well, and that they recommend schools should actually start at 8:30 am or later, and most other evidence points in the same direction.
Thus, for the sake of everyone's sanity, start school later.