Well no actually, the US Constitution does not prohibit teachers or school officials from promoting religion, it only prohibits Congress from establishing a national religion.<quoted text>
You misunderstood. Constitutional law as interpreted by the Supreme Court forbids school officials from promoting religion or leading prayers, but it also protects the rights of students to practice their faiths during school hours as long as it doesn't interfere with school activities or impinge on the rights of other students. Disruptive behavior is governed by school policy, the religious rights of the students by constitutional law. Which one applies depends on the situation.
The conversation, then, involves both constitutional law and school policy. There's no "switch," only a view that encompasses both and applies each where appropriate.
There are school policies about disruption in class, and I could honestly see how a prayer could be seen as a disruption. But I could also see how texting or shuffling cards or farting could be disruptive. None of those following the guidelines of the US Constitution.