Working off of memory so take this with a good amount of salt. If I recall correctly, studies have found that teenagers generally don't go to bed later to match if school starts later, but the same applies the other way too. It may be that they technically could get that sleep by going to bed earlier, but as a society that is not a viable proposal to fix the problems that arise from their lack of sleep.

Also just to make sure I'm not misunderstood, I don't dispute that individuals can get used to a new sleep schedule or will go to bed earlier if they have to get up earlier, just that it isn't as easy as waking two hours earlier, means go to sleep two hours earlier for teenagers especially.