When a teacher watches a teacher - good!
Once a week each teacher in each school should have one period free during which time that teacher would observe the teaching of another teacher. Not as a critic, or as part of an evaluation program.
But would watch to learn - to pick up ideas for improving his or her own teaching.
In a community with many schools teachers might actually leave their own building and visit another during that observation period.
And certainly, if there is a nearby college or university, K-12 teachers might well enjoy and be inspired by the different teaching methods and styles they would have to draw on.
What about the teacher's own class - the one he or she is leaving during this period of observation?
Class members could be in a language laboratory; or in the library; or at gym; or under the guidance of a team teacher; or supervised by an aide; or have the principal or another administrator as a substitute teacher.
But a once-a-week observation time, written into each teacher's schedule, would put all teachers on notice that teaching techniques can (and should) be improved and that the school district citizens think that the art and craft of teaching are so important they are willing for every teacher - at least once each school week - to have the opportunity to learn how to improve.