Do you know that said warning had not been already given and this wasn't "next time"?
Do you know that the teacher had not clearly stated to the class what the penalties would be for the wrong paper size?
In either case I can see the teacher acting as described. And wouldn't fault the teacher for doing so. If, indeed, the teacher was enforcing an arbitrary rule which had not been clearly communicated, then I would fault the teacher. But I suspect this was not the case. The rule was present and the kid was supposed to know it, but forgot.
Do you know that the teacher had not clearly stated to the class what the penalties would be for the wrong paper size?
In either case I can see the teacher acting as described. And wouldn't fault the teacher for doing so. If, indeed, the teacher was enforcing an arbitrary rule which had not been clearly communicated, then I would fault the teacher. But I suspect this was not the case. The rule was present and the kid was supposed to know it, but forgot.