I agree, teachers out an about in public have a certain responsibility to maintain their ethical conduct in the presence of their students. When that presence is incidental, that responsibility becomes somewhat murky and subjective, but I think it's a generality that holds water.
So really what the ACLU is doing here (no surprise) is using the "official capacity" doctrine of the "consent decree" as a completely disingenuous way to attack Christianity and freedom of religious expression under the 1st Amendment.