In one class a few months ago, I had a couple of students who were probably using a thesaurus. One consistently used the word "gatherings" for "parties" in reference to a party in a legal case. I brought it to the attention of the student in my grading for the first few weeks but it continued. I graded accordingly explaining that a gathering is not a legal term. The two students, I suspect from their overall grammar, were not native speakers. 

If you suspect it is true plagiarism, not a student not understanding what they are writing about, try changing the words to what they should be (student or learner or NCLB) and run a sentence through Google. If it is taken from a source, it will likely show up in the first entry. 

I also have mentioned to students that the work needs to be in their own words to show an understanding of the topic. I am currently finding a few students who cut and paste quotes (with quotation marks and citations) from the readings to answer the Discussion Questions. I mark down for that and note why in my comments section for the grade. 

I wish you the best in resolving this but I remember back to being a student all those years ago and taking classes I needed to but didn't want to. I can't imagine I turned in my best work all the time. No, I didn't copy, but I know I could have done more in some classes! Ah, to be able to redo college. Yeah, nevermind. LOL