agree with Oldlady.

I think if the police are involved, the parents sould be notified, but not if the school are dealing with it internally.

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?

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.

If a high school student witnesses a incident between another student and teacher where the teacher abused a student -

Before having this student(witness) questioned or made to write a report-
Should the school:
Inform parents?

Does the school have the authority to remove a student(witness) from class to question him about his statement - without notifing parents?