IMO it started with no child left behind, and a wave of public sentiment and legal precedent that places the worst performing and behaving students above the majority.
I have a teacher friends who have to deal with students attacking them. One had a student break their hand and they could not suspend or expell them. Imagine trying to teach a class under such conditions.
That is not mutually exclusive. First, you can absolutely sacrifice the median to improve the average. Second, I was pointing out when I think it started. One hypothesis is that the negative impacts of no child left behind and similar programs were initially mitigated/compartmentalized by the heavy academic tracking used at the time. My understanding is that much of this has changed over time with a reduction in number of tracks.
I have a teacher friends who have to deal with students attacking them. One had a student break their hand and they could not suspend or expell them. Imagine trying to teach a class under such conditions.