Not sure if they have the authority to do so or not, but that is certainly the case at my school. Time is often of the essence and the sooner you remove a witness from a classroom and away from other people, who may sway them one way or another, the sooner it can be dealt with. I often have students sitting in my office, so that they are away from other students who may be making a statement.
Depends very much on the situation I think - but I would certainly expect to be informed if my child was involved in such an incident , however I can see that this would quite possibly occur after the event if the situation needed to be dealt with immediately, IYSWIM. Can you elaborate, OP, is this something which has occured with your DS/DD?