There is overwhelming data showing teens need more sleep. When high schools start at 7:00 AM many students are not fully prepared to start their day.
My son attends Central High School and getting up at 6:00 is quite difficult. He has trouble going to sleep earlier. Check out this recent New York Times article: http://well.blogs.nytimes.com/2014/03/13/to-keep-teenagers-alert-schools-let-them-sleep-in/?_php=true&_type=blogs&_r=0