They can refuse to let the child leave. Schools have all kinds of guidelines and rules under which a teacher/other school personnel will just keep a child in a classroom/in the office until everyone has to leave. At that point, they might very well call one or another authority.

It's all very well to say, "They can't enforce that rule," but if your answer is making a kid who is already in trouble just walk out in defiance of rules the parent has certainly been informed of, I don't see that ending well at ALL.