When I was your age, I needed a minimum of 9 most nights. If I had less than 9, I could do it or two nights and then I needed to catch up. I remember this well because I went to the doctor a few times complaining about being tired (and therefore cranky) all the time. Some people just need more sleep. Try going to bed earlier, if possible, and getting all your stuff ready for work the night before so you can sleep later in the morning.
Now that I'm older, I am okay with 7 but I try to get 8 if I can. I'm a night owl so that's tough.