I believe that teens need between 8 and 10 hours of sleep each night. Perhaps you misunderstood the 10 and 12 reference.....?
I believe that you should listen to your own body's clock when it comes to sleep. There might be times where all you do is sleep and go to school. That might signal you're going through a growth spurt. If you function better when you go to bed late and wake up early and that works for you, then go with that.
If you need to adjust your sleep time sometimes, that's ok. If you're having trouble falling asleep, be sure all electronics are off (it's the light from these that disrupt sleep). Hope this helps.