I found the statistics in Dr. Matthew Walker's Why We Sleep [1] pretty compelling.

Starting an hour layer improves SAT scores:

One of the first
test cases happened in the township of Edina, Minnesota. Here, school start times for teenagers
were shifted from 7:25 a.m. to 8:30 a.m. More striking than the forty-three minutes of extra sleep
that these teens reported getting was the change in academic performance, indexed using a
standardized measure called the Scholastic Assessment Test, or SAT.

In the year before this time change, the average verbal SAT scores of the top-performing
students was a very respectable 605. The following year, after switching to an 8:30 a.m. start time,
that score rose to an average 761 for the same top-tier bracket of students. Math SAT scores also
improved, increasing from an average of 683 in the year prior to the time change, to 739 in the
year after. Add this all up, and you see that investing in delaying school start times—allowing
students more sleep and better alignment with their unchangeable biological rhythms—returned
a net SAT profit of 212 points. That improvement will change which tier of university those
teenagers go to, potentially altering their subsequent life trajectories as a consequence.

(This is one example, that has been replicated many times, he covers more in the book.)

It saves even saves lives:

Yet something even more profound has happened in this ongoing story of later school start
times—something that researchers did not anticipate: the life expectancy of students increased. The leading cause of death among teenagers is road traffic accidents, and in this regard, even
the slightest dose of insufficient sleep can have marked consequences, as we have discussed.
When the Mahtomedi School District of Minnesota pushed their school start time from 7:30 to
8:00 a.m., there was a 60 percent reduction in traffic accidents in drivers sixteen to eighteen years
of age. Teton County in Wyoming enacted an even more dramatic change in school start time,
shifting from a 7:35 a.m. bell to a far more biologically reasonable one of 8:55 a.m. The result was
astonishing—a 70 percent reduction in traffic accidents in sixteen- to eighteen-year-old drivers.

To place that in context, the advent of anti-lock brake technology (ABS)—which prevents the
wheels of a car from seizing up under hard braking, allowing the driver to still maneuver the
vehicle—reduced accident rates by around 20 to 25 percent. It was deemed a revolution. Here is a
simple biological factor—sufficient sleep—that will drop accident rates by more than double that
amount in our teens.

There's more reasons, like improving attendance and decreasing drug/alcohol use, but these are the ones that stuck out to me. There was another statistic that I can't immediately find a quotation for that was pretty mind-blowing for me, which was that in university, the difference in performance in controls and students who started class an hour later was equivalent to the difference between controls and students who had a professor a standard deviation above the average. They even showed that this was dose dependent (an extra hour later had more effect). I think it's pretty incredible, considering the skill gap between an average professor and a great one, that simply taking a class at a 10am instead of 8am can increase learning so much.

[1] https://www.amazon.com/Why-We-Sleep-Unlocking-Dreams/dp/1501...