Just tuned in.
I don't know when vaccinations to go to school became optional.
In the 50's when I was in "grade school" in Salt Lake City, the whole school population was part of the testing of the Polio Vaccine.
Half the population were vaccinated with the vaccine, and half were the test group that got a vaccination, but without the vaccine.
The next year after the vaccine was accepted, the group in the test group got vaccinated again with the real vaccine (Yep, I was one of those.)
There wasn't a choice, other than leaving, that I was allowed.
And yeah, after mandatory vaccinations of all school kids, Polio disappeared for a long time.