Our eldest DD (now aged 12) started senior school last year and I had similar concerns to you OP. School starts at 8:30, but the bus leaves at 8 from a stop a 15 minute walk away, so I had anticipated her leaving the house at 7:40 after being woken by her alarm at 7am. We practised one morning before term started and got to the bus stop with 5 minutes to spare, which I thought was ideal...
The reality has actually been very different - it appears that DD1 simply cannot bear the feeling of running late at all and therefore prefers to leave the house at 7:30am following a 6:15 alarm. She showers in the evening rather than the mornings and has her bag packed and everything ready for school, so it's simply a case of getting dressed, hair and teeth brushed, face washed and making/eating breakfast which should really only take a maximum of 30 minutes, but she just prefers a more leisurely, relaxed approach to the start of the day. On the one occasion when she'd not set her alarm and woke up at 7, her panic and distress was overly dramatic awful for her and I ended up driving her to school (despite me driving past the bus stop 5 minutes before the bus was due!)
If it were me, I'd set my alarm for 7, have two one 10 minute snooze, quick shower, hair, make up, clothes and eat a piece of toast on the walk to the bus after leaving the house at 7:40ish - I think it just depends on the type of 'morning' person the individual is.
I'd recommend having a couple of practices before term starts and gauging how fast/slow your DC walks when they're on their own. They'll soon get into the swing of it.
In terms of actually waking up, the first couple of weeks DD1 was really excited to be at a new school, so there wasn't an issue with getting her out of bed and now that she knows how much she hates being (or even the feeling of running a little) late, she's actually pretty motivated to get moving in the morning. Although we started last Autumn term with a 'lights out' time of 9:45pm, she found it quite difficult to fall asleep and laying in the dark, waiting to fall asleep seemed to just end up with her becoming anxious that she would never get to sleep, so we now let her read until she falls asleep (I understand this as I'm the same, I'm forever waking myself up after dropping my book on the floor or my face!). Obviously every child (and adult) is different and unless your DC is great at going straight to sleep at a set time, it may just be a case of trial and error till you find the right time for them to have lights out whilst ensuring they get enough sleep - of course the first few weeks of Senior School are pretty exhausting anyway, so your DC may be ready to fall asleep by 7:30pm!
I'm sure you'll all quickly get used to the new routine, but good luck!