Not 7-9, 8-10 hours of sleep is recommended for children ages 13-18. In addition 7 out of 10 high school students do not get enough sleep. The amount of sleep that is advised for teenagers is 9 1/4 hours (which is more than 9 hours, as some people so want you to believe).
So if you are a teenager and sleep 10 hours a night, contrary to what some people want you to believe, 10 hours is actually very healthy for you. I'd say it's important to look at good studies on the topic, rather than what a random internet stranger from another continent wants to tell you though, so you should do some research yourself too.