Table 7 in the 2018 Superintendent’s Annual Report gives us, among other things, a count of the number of public school students retained in grade from 2017. The short answer: A few in the first two grades and then hardly any until high school.
Exprerssed as the percentage of students, the graph looks much the same:
This greasing of the academic skids until the student reaches high school, but only until then, surely is related to the “ninth grade bump” that looks to be a nationwide phenomenon.
The principal argument for social promotion seems to be that holding the kid back is even less effective for learning than promoting that student. The literature I’ve seen does not explain why that reason (or simple inertia or moving the problem on to the next level or whatever else might explain social promotion) stops at the ninth grade.