A couple of detentions for repeated lateness, plus being kept behind everyday to make up the time they're missing, tend to have the effect of concentrating their minds and making them more amenable to trying something different.
In addition, waking them at the same time at the weekends, using sunrise alarms and making sure that they have got everything ready (uniforms, bags packed, etc) the night before encourages them to feel tired at the same time each night.
I'd also think about working in threes (or 90 minutes) - even teenagers sleep in roughly 90 minute cycles, so it might be that you're trying to wake them right in their deepest sleep; for example, if they normally sleep by 12 and struggle to get up at 7, wake them at 6 when they are more likely to be in light sleep. After a few days, they're likely to start going to sleep earlier - or at least coping with the 6am start better than the 7am one. If you feel particularly kind, you could then adjust the 6am Saturday and Sunday wakeup to 7.30am, thus giving them an extra sleep cycle.
I didn't realise that my sleep cycles are so fixed, thanks to chronic insomnia. Turns out that getting up early is way better for me than staying asleep and having to drag myself out of bed to stagger around looking for things. Which is why I now start work so early - I thought a later start would be better, but it really isn't; if I don't sleep, as long as I have a multiple of 3 hours or 90 minutes, I'm fine, but I usually go to sleep three hours earlier than what was, until this year, the earliest I would ever go to sleep.
Other than that, no TV at all in the morning if they like doing that (or if you do) and no phones until they're out the door. Less distraction from what they are supposed to be doing that way.