Teenagers need lots of sleep. And, when the seasons change or they are fighting off an illness or infection, they need even more. The best thing to do to figure it out is to go to bed on a weeknight as soon as you feel sleepy and don't set your alarm. Just wake up naturally, see how many hours it was. After you try that a few times, you will be able to figure out the average time you need to sleep. Then, you can set your alarm for school and go to bed and wake up according to the needs for your health.