There's really not set amount of hours that a person or teenage sleeps so sleeping longer or shorter hours can be normal.
Some teenagers only need to sleep for 9 to 10 hours while some teenagers need 12 hours of sleep so it can be normal for the teenager to sleep for 12 hours if they need the sleep.
Sometimes the teenager may also just be overly tired as a result of not getting enough sleep such as on the weekdays when getting up for school or staying up at night for studying etc.
Then the teenager may need to sleep for 12 hours to make up for the sleep they were deprived of so there's nothing wrong with the teenager if they need to sleep for 12 hours.