It is generally considered that 8 hours of sleep a night is insufficient for teens, particularly while they're still growing. There has been a lot of research about the importance of early-morning sleep for teens' brains to function properly, particularly their memories -- but that's hardly useful information if they have somewhere to be before 7am.
In some northern European country, probably Sweden or Norway, the school system took this research seriously and swapped high school to start at 1pm. I'm sure your kids would love that...
Of course, I think of things like time zones... if they were living in a time zone 8 hours away, sleeping to 3pm would be getting up at 7am... and to my mind the issue is still duration, not the time of the day.
Our culture has made a big deal about the sophistication of 'staying up late' and how 'babies go to bed early' (and nap)... when that doesn't reflect actual sleep needs very well. If a child needs 12 hours of sleep in the course of 24 hours, they should sleep for 12 hours, starting early enough to actually get enough, or napping earlier to enable them to go to bed later. Perhaps a nap after school would do more for their ability to wake on time than going to bed 2 hours earlier (which is probably still not early enough).
It has been suggested by sleep researchers that North Americans on average are underslept every single night by more than an hour and a half... which adds up (as premature aging, body stress, as well as eroding memory and concentration) over the week to a deficit of 8-15 hours... the equivalent of not having slept at all for one or more nights. This impairs reaction time and has been suggested to be as dangerous for driving as being significantly drunk.
What will make it easier for your kids to wake up on time? Getting a little older, no longer growing, getting enough rest throughout the day, getting enough sleep before midnight to enable them to wake refreshed and some variety of stress-relief strategy so they sleep better all night. Meditation, daily exercise, prayer, massage, avoiding stressors (like tv and monitor screens -- the flicker aggitates brains) late in the evening, hot baths, turning down the lights so the evenings are dimmer...