I taught school in a very small community where we had the pledge, and moment of silence daily. I was obviously cool with that because I am Christian and attend church most Sundays. There is my disclaimer for the following statement.
Religon in school is usually a touchy subject due to those individuals that are looking for a cause and or a soapbox to stand on while their "real" issues dont hold water. That being said, its almost easier to allow for the school's to determine how they address this issue as they see fit. The responsibility then falls on the Admin, school board, ect...
The line that defines this idea is "who" is doing the preaching, and "what" are they preaching about.
Case in point: I had a substitue come to my class one day and when I returned I discovered he had preached his beliefs and basically gave his testimony to them all. That's a "no-no" in Texas public schools. I then find out he was Church of Christ and made comments about me being Catholic and the Catholic faith in general. Apparently, I'm not going to heaven according to him. That's a "no-no" with me.
If a liscensed teacher cant do it, some substitue has no right to.