A 15 year old needs a minimum of 8 hours of sleep per night to function properly the next day and to stay healthy.
Some 15 year olds sleep 8 hours per night and other 15 year olds may need to sleep up to 9 to 10 hours per night depending on the individual.
When I was 15 years old which was many years ago now I used to sleep up to 10 hours per night on the weekends and on weekdays I would sleep only 8 hours per night.
But then again I would sometimes stay up all night and then be so tired the next day.
I wished I'd never stayed up all night even though it seemed fun at the time.
The next day was not fun at all.
But as long as the 15 year old is getting at least 8 hours of sleep per night then they are doing fine but some 15 year olds may only need 7 hours of sleep.
It all depends on the person.