The Baby Boomers were pretty much out of the public school system by the early 1980s. This is a very recent drop. My thought is that it is from parents home schooling or sending children to private schools because they are afraid the public school teachers will try to transgender their children without parental knowledge.
There was a drop in the population attending public schools and it didn't really recover until the late 1990s (legal and illegal immigration). So we had a noticeable drop for 20 years. Now this is another drop (likely to be permanent unless we vote for another Democrat).