... will be threatened by kids in school? ... and the schools have a responsibility to prevent this unlikely transmission vector?The younger children who are not in school yet and not able to be vaccinated yet will all be threatened.
Being vaccinated does not mean you can't carry measles.
MMR vaccine is administered very early on. My kids got them at something like four months.
By the way, I was part of the first cohort to receive the newly developed Polio vaccine, way back when and I still remember a number of people who were stricken and crippled for life by it. I also had measles and a temperature of 104°F and I still remember what it was like 55 years on. Keeping me out of school would not have prevented me from contracting measles. It might have made it slightly less likely but the first time in public ...
The schools have gone too far. Some other authority can kick children about because of the decisions their parents make. The kids rights are being violated by the schools.