But this study is asserting that the earliest 8am class, of a system that runs 8am to 4pm, has the worst performance.
It can’t say that a 9am class in a system of 9am to 5pm would have better performance at 9am, and it certainly can’t say that the system as a whole would have better performance. Maybe 5pm is too late and affects child care or dinner or night jobs and now students are distracted all over again.