a decrease in the American population.
If only.
As to everything else, it's not that PZ is dying, but minecraft, the generation that first spawned Minecraft's popularity is getting older and leaving it behind.
And while we did have an influx of in and out each year as people lost interest and new people gained interest, newer better games are coming out, as well as Minecrosofts new updates trying their best to murder MC has kinda started to cool off the major explosion Minecraft had.
However party games is another possibility here, as the PZ clientele was slightly older than your average minecrafter, and party games are a cliche not only from other major servers, but from the unholy Nintendo zombie itself, Mario.
Things like that can really ruin a reputation, and some of the older players may decide to leave because of this, increasing the outflow of player.
(Not to mention change is scary, and that helps too).
Not to mention, most of the regulars here are now old enough to get jobs, and with a lot of summer openings showing up for seasonal positions, people are going to start earning money for spending on real things, not a virtual world, and with time being scarce as it is, minecraft gets moved to the bottom of the priority list.
(Especially where I live, everyone is hiring anyone they can get).
All this combined with school testing is keeping server numbers down.
Tl;Dr It's probably a mix of things, not just school testing.