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I think sight of what "plagerism" means is being lost here.
Teachers will get upset if you do *not* plagiarize.
This implies that the pupil is expected to quote from one or more, known (how else would the teacher know that the work was not original?), authoratative sources.
Quoting from authoratative sources does not fall under the definition of plagerism, the essence of which can be summed up by:
Taking, using, and passing off as your own, the ideas or words or work of another.
If the teacher is expecting it to happen, then when it does, they will know it has, because they will recognise the source. There is no deception or misrepresentation involved and therefore the term plagerism does not apply. Even if the reference is uncited, how can it be plagerism if the teacher is expecting it?
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.