That’s not necessarily true. My brother teaches in a public school and the teachers do not discuss where they stand politically with their students. I asked him if that was mandated by the school board and he said that was a decision made by the teachers themselves. He said the teachers don’t even discuss it among themselves.
If true, that is an outlier.. even if most teachers in a school are willing to be professionals, anymore there it is rare a school does not have more than 1 activist teacher who will flagrantly push their beliefs upon their students... and when they do, they are not called to account for it.