I don't mean to nitpick, but I don't think it takes very long to break a sleep habit. I travel quite a bit and so am not only changing my actual sleep hours significantly (i.e. going to bed at what used to be my 4pm or whatever) but also changing them based on local timezones (so this job requires me to go to bed 2am local time, vs another job where I can go to bed 10pm local time). Anyway point is I'm usually adjusted within a week.
And I certainly adjusted from my "classes start at 7am" in highschool schedule to my "classes start at 12pm" schedule in college near instantaneously.
Your personal anecdotes say nothing useful about the experiences of other people.
I get pretty wiped out for a week when the clocks go forward or back. I'm only marginally productive when the sun is up, but get a huge amount done from about 8pm to 2am.
But so what? That's me. I don't expect the rest of the world to be the same.
Sleep is clearly very individual. Standard school/office hours are a hugely inefficient waste of the potential of tens of millions of people.
It should be possible to design a more flexible system. But we still have ridiculous notions that early risers are somehow the model of virtue, and everyone else should aspire to the same hours.
I've always found the best way to deal with jet lag is just to stay up until it's a normal time to go to bed in one's current timezone. Depending on where you are going/arriving from this can be quite a long stretch, but it generally gets me set on the local timezone within about two days.
It helps that I'm not very good at sleeping on airplanes unless I've been awake for >30 hours
It also helps to pre-empt it a little. The last few times I've flown long-distance, I've tried to adjust my sleeping a day or two in advance of flying and had very little problems.
And I certainly adjusted from my "classes start at 7am" in highschool schedule to my "classes start at 12pm" schedule in college near instantaneously.