Sleep needs don’t change all that dramatically from late elementary through middle school into high school. What changes is the circadian rhythm of sleep and wake, and typically as you go into and through puberty your sleep and wake time shifts by as much as two hours. Teens simply can’t fall asleep as early as they did when they were 7 or 8 years old. That is why many experts say the high school day should start later.
How about, say, 9:00 to 4:00?