That process started in the 1930s with the importation of the Frankfurt School into Teachers College. By that time public schooling was such a large enterprise that it took generations for that communist ideology with all of its obvious baggage to overcome institutional inertia. But it obviously now has.
Now the leftardian education IS the institutional inertia that has to be fought against. It will take more generations. In the interim, parents’ educational cooperatives are gaining a foothold, as is homeschooling and parochial schooling.
It did not, you are way too late in your timeline. That process started with Horace Mann himself in the 19th century. That's why Massachusetts is often times arguably the furthest along when it comes to radically leftist proposals.
In Mass. they've been indoctrinated for the longest period of time in the Horace Mann schools.