No less than 9. The growing teen is faced with many choices but needs to take care of his/her sleep if they are to function well. While growing, no less than 9 hours of sleep/24hrs is needed. After their linear growth has stopped, they can sometimes get by on 8. Some will need more based on sports participation or heavy study. Events like sleep walking and sleep talking are evidence they're sleep deprived.
8-10. Ideally, 8-10 hours a night.