The question seems to be asking why the Seniors aren't still running after crossing the finish line? Those schools doing this, seem to have adjusted their schedule to state mandates. Even in my day, I don't think it was unheard of for Seniors to have a different end-of-year schedule than the rest of the population. Is it possibly just new awareness to you?
As for the rest of the population, I don't know now. In the day, there was concern the state tests covering material not normally reached by the median students until after the test dates. This particularly hurt ratings of schools with lower performing populations (for whatever reasons) that needed the entire year to reach those points with their students. None of this mattered to state until the "Bexley's" started complaining. I suppose curriculum was moved/sped up in response to trying to get those state points.
Ex: The Standards in HS Geo were five points. Three covered by about seven chapters of most texts and proportionally that part of the school year. One standard covered by three chapters. And one, Stats, which had absolutely no prerequisite in the other four points was the remaining point, every bit as valuable towards district evaluation as the others. Stats was generally, last chapter of the text and not reached by many teaching traditionally as opposed to the new pragmatism. They were concentrating on Geo. Smart people realized, move the Stats up in the year, save three chapters till the end. And if the kid has already taken and passed the state test, what motivation do they have, to learn those last bits? They've reached the finish according to State.
That's one subject out of several. Can you imagine detached Columbus managing any different on those others?
Much of the end of year is traditionally used for ceremonies, events, fun, community building... anyhow.