Please let them sleep!
I remember all of the times I went to WDW with my mother, who is notorious for waking up at 6 AM every morning of a vacation.
When she'd try to prod me out of bed at 7 or 8 AM, it really made me cranky. Remember, the teens are on vacation too! Waking up at the crack of dawn only incites the feeling of getting ready for school, and who wants to be on that wavelength while at Disney? In the mornings I'd be tired and grumpy, and even though that feeling would dissipate when I got to the parks, it would return in even fuller force the next morning.
I say, let them choose when they want to get up; if they see after the first day that it's not worth it due to the long lines, then they'll probably ask you to wake them up the following day. However, they may be fine with it (I always was!), and pick being well-rested over getting an hour of wait-less rides.