…even if it were the case that schoolchildren's sleep cycles were dictated by the sun (which several comments in this section indicate isn't the case, and contain references which I suppose one could follow) … my school, for example, started at 7:50am. That meant waking up well before sunrise during some parts of the year (~70 minutes before sunrise, in the darkest portion¹). I remember always being very tired for first period … all the more amazing that I passed AP Physics, which I had first period.

¹(sunrise ~7:11a in the winter, we'd wake up at 6a. ~40m for breakfast/morning routine, ~1h for transit, and a little bit of wiggle, since you'd want to be there by ~7:40a at the latest.)

(… and there are routinely threads on HN where people advocate for moving that wakeup to ~5a standard by futzing with DST.)