As someone who lives in a country where schools all uniformly started at 9:00 am (i'm probably wrong on that, but lets just go with it), what's the social-background/theoretical reason for starting the school day at 7:00am?
From the outside that just seems absolutely bonkers?
As noted, bus scheduling. Because the US doesn't have a robust public transit system in most regions, it relies on dedicated school buses.
The really goofy thing is the schools are traditionally backwards. High school starts earliest (call it 7:30am), then middle school (call it 8:15am), and then elementary school (9am).
Physiologically, that should be reversed, as teenagers have a harder time getting enough sleep and functioning in the morning.
Timing-wise, starting ES earlier works out better for many parents, as a 9am school drop-off is too late for them to start their commute. So, one parent ends up time-shifting their work day in one direction (to drop off or walk to bus stop), and the other parent time shifts the other direction (to be home at the end of the school day). Failing that, the kid gets sent to in-school day-care before and/or after school.
If ES started at 7:30am, both parents could start their day at the same time, and only worry about child care after school (if neither can be home mid-afternoon).
Or, we could just stop sprawling all over the darn place, build some transit, and start all the school at whatever time makes sense for those students.
Remember that 7am in one place is not the same as 7am in another place. Time zones, latitude, seasons combine to cause big differences in what a time "means".
For instance on the equator sunrise and sunset is very consistent and so people tend to wake up earlier so that they can maximise daylight. Closer to the poles, it makes
sense for school days (which tend to be shorter than work days) to start around the time that the sun rises in the winter so that they are in daylight all year. This is all relative what error your time zone has, as shown in this famous map:
There are two distinct equators which are not equal: celestial (perpendicular towards the axis of planet's spin) and ecliptic (a circle from planet's sphere on a same plane with orbit from a celestial body). On Earth they differ for 23°44' now (it's changing over time) which is why we have distinct seasons. As default we talk about celestial poles and equator which isn't right for theme of sunlight.
Also the "famous" map is outdated at least by 5 years, Russia has switched from permanent summer time to permanent standard time in 2014. Wikimedia has an version[0] which is somewhat maintained.
Not knowing the real reason, I suspect it's so that parents can drive their children to school before starting their commutes to work. What proportion of kids do you reckon walk/bike/transit to school in your country?
Honestly (Aus), relatively few. Drop-offs would traditionally be from about 8:00 am onwards (so kids had up to an hour before hand waiting for classes to start). Not everyone arrived from 8:00, but getting there well before that was exceptional.
We don't generally have "dedicated" buses for school, though there are some schools that have dedicated routes at certain times of the days usually contracted out to bus companies, otherwise its on the parents or the public transport system to get children to and from schools.
When I was younger (sub-12 years old, both parents working), parents dropped me off on the way to work, then we had a kind of neighbourhood arrangement with other parents at the school where we'd be picked up by them (car-pooling i guess) and go and spend the afternoon at their place before my parents picked us up from their place on the way home.
For high-schoolers, its a bit "well, they're capable of getting themselves to and from school by now...", with obvious exceptions if you live out in the sticks or miles away from the school (then the question would be, why are you going to that school without a way to get there?) and no real stigma to it if you can sync up with parents or after-school activities.
You know, i'm much older now, so its possible I'm out of touch, but I doubt I'm THAT out of touch to make a 7:00 starting time seem vaguely sane...
Edit: I've looked into it and I'm not that out of touch. State recommendations where I live are from 8:30 onwards, with most starting at 9:00.
The part about this logic that confuses me is starting early enough to early morning commutes (7ish) puts kids out of school long before parents are home (2-3ish). I remember briefly running with a crowd's whose daily ritual was, after being dismissed at I think 2:40, was to go home and smoke pot for a few hours before everyone's parents were home. Unless kids are in school for 9+ hours, I don't see how parental commute is a reasonable argument. Similarly, there's no reason there couldn't be early classes for kids who absolutely couldn't get to school otherwise.
Another perspective from Australia. When I was in high school virtually everyone took the bus to school except a few of the yr 12s who could drive themselves. The only problem was the nearest bus stop was an hours walk away from my house so I still had to get driven to the bus stop.
From the outside that just seems absolutely bonkers?