Though I haven't used it, I do remember some posts about Part Reviewer.
A workflow could be to open two instances of Solidworks and then step through the two parts and see if they were created exactly the same.
If you are concerned about this in the future, it may be a good tool to prevent future plagiarism by dictating that the students comment on their steps using Part Reviewer. And then the comparisons may be a little easier...Maybe not, as they could change the comments, but it may be worth a try.
I wrote a similar application for a customer where he wanted to get the details of every features of a model extracted to an excel file.
Here is link of video showing some functionality of that program: SOLIDWORKS Files Details as Excel Report
I helped out a university lecturer with this a while back. He dealt mostly in drawings and found the draw compare tool to be really handy. Basically it will show if the drawing views/dimension etc haven't moved a .mm which is really unlikely between two separate students work!