Making a distinction between "that was X" and "you are an X boy" is a shibboleth which apparently the teacher doesn't share. I think it would be ridiculous to follow it up. Paradoxically, insisting that these are very different is itself to encourage a fixed mindset. At that moment, he was a rude boy, because he was doing a rude thing. Doesn't follow that he was a rude boy five minutes later.

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