Thanks to all the people who passed comment about natural levels of organisation. I am up early (usually an hour before the kids), and everything is put out the night before. I always used to be well on time for everything, and have no problem planning my own time, but the mornings are just a disaster....
Reading through the more helpful suggestions (thank you), I think I am pinpointing my personal problem to the leaving-the-house bit (now that I have accepted, grudgingly, that my 9YO has to be physically dressed by me in order to get him out of bed in a timely fashion), as all the other stuff tends to happen quite neatly: once they're dressed, there's no faffing over breakfast or teeth-brushing. It's the putting on shoes and coats, picking up school bags, and getting in the car (or onto bikes, depending on the day) that causes the problem.
On the days we walk to school (Tuesdays and some Thursdays) we are normally on time, or if not on time, then at least not late (in that magic five minutes between the bell ringing and the register starting) - mainly, I think, because if you are late when you're walking, you can always RUN to catch up the time (whereas the car journey is always going to take as long as it takes and there's very little one can do to influence it).
Now I just need to think of some way of getting them to understand that yes, we DO need to start leaving the house that early, however much they may think I am allowing a ridiculous amount of time. I had been thinking that the answer was to mount a highly visible clock somewhere (we have several, but I was thinking maybe they weren't "in-your-face" enough), but I'm actually beginning to wonder whether the answer is not to make reference to the time because they have got it into their heads that they need to start leaving later than in fact we do.
New idea: I'm going to try setting an alarm clock for 'leaving the house time', preferably one of those loud old-fashioned ones that sounds like a fire alarm. Will report back on how that goes.....