I think it can be tricky to know the difference between plagiarism and research/attributing sources.  Does one copy just a sentence from a source to make a point, two sentences, or an entire paragraph?  If an entire paragraph, then why not the whole paper?  It's a slippery slope, between the two.  I had trouble with this in high school myself.   There's an art to quoting only a sentence or perhaps two sentences from a source, and incorporating it into one's paper without plagiarism.  It's hard to know the limits - what's too few source material? What's too much source material?  What is a work of creative fiction invented by the student, and what is plagiarism?  -Kenneth