It would be impossible to describe all the ways in which this is stupid. No doubt, someone is saying that "if it makes just one child safer ...", but I can't imagine that cameras in the classroom are a cost effective means of doing that. As for interfering with teaching, I doubt that a teacher who minds being watched would be any better without the cameras. I've had quite a few visitors in my classroom, and except for occasionally asking them to be quieter (they get into loud, off-topic conversations with each other, and I don't permit that from my students or from my guests) I ignore them and don't do anything differently when I'm being watched. I like having cameras in the halls and outside, to catch the occasional vandal, but the number required for classrooms is too high for the cost to be reasonable.
I’ve installed hundreds in schools and except for gyms and computer labs none of them are in classrooms.
I don’t think the teacher unions allow it. As you said they are most useful in catching vandals and thieves as well as they occasional assault.