I would say the world simply agrees that things - whether schools, shifts or parties - should generally start sometime, and thus, by necessity, be less accommodating to some people than to others.

1) There are social, economic, and institutional norms surrounding sleep.

2) These norms are defined (or at least abided) by a majority group.

3) A minority of individuals can’t abide by these norms either due to physiological differences or necessity.

4) Some people benefit from the work that this minority does, and the minority is not adequately compensated in comparison to the benefits they generate.

It was my first impression as well, but if I get over my gag reflex and actually read the lines carefully, it seems the "injustice" basically means that people with different schedules are treated unfairly by society and the "minorities" are precisely those with different schedules.

In the suburbs I'm familiar with it was 100% a bus scheduling constraint. The high schools had to start at the crack of dawn because by the time the same fleet of busses made it through the middle schools and on to the elementaries it was getting pretty late.

You could alternatively argue that it was a budget constraint (not enough money to 3x the bus fleet). Either way, it sure would have been nice to have robust city planning and public transit (and thus not need a large private bus fleet in the first place).