This semester I had to deal for the first time with a case of what seemed to me pretty deliberate plagiarism. It caused me to think a bit about plagiarism, which I really haven't given much thought to before.
In fiction, there are three levels, it seems to me. 1) To use an already existing fictional situation. This isn't that serious because most situations can be reconfigured so as not to rely much on the original, so it's more like taking someone's idea and running with it. However, certain fictional depictions kind of trump further use of a situation without incurring debt. We have many updates or re-imaginings of already existing material -- like the film Clueless's treatment of Emma -- but it does necessitate acknowledgment. 2) To use deliberate incidents from prior depictions; this is when the plot points or the incidental characters or a particular line of thought are employed in the same sequence or with the same inferences as in an original. Now we're getting into real trouble because the sequence of events, how details are used, and how actions play out are largely what fiction consists of, and these things have to be protected as original to the writer who uses such material initially. 3) To use actual language from the published text. This is the worst of all. The choice of words is what is most deliberate and individual to each writer. To import the words of others without treating them as quotations or allusions violates all the expectations we have about reading something "in the original."
I feel that unacknowledged use of published material in a creative writing course is the worst kind of unacknowledged use of published material that can occur in a college course. Ideas and the statement of facts should always be traced to where they originate, if possible, but both are able to be restated in various ways so as to avoid outright plagiarism. The ideas in creative writing are dependent on situations depicted and the very words in which they are described. A published work can be referenced so as to lead the reader to the original, in which case the allusions act as commentary. The original is indicated. But to treat work one has read as material one has created or invented is a travesty of the process of creative thought and a serious infraction of the code that defends original material from theft.