Finding duplicates from submitted files

XnView has a find duplicate function that calculates a CRC for each images and matches against it. XnView, IrfanView and of course Bridge allow complex searches based on multiple criteria, so you have plenty of options. I'm sure there's even more tools out there...

Mylenium

Much depends upon budget, the number of students in the class and their ability. My classes were limited to 15 and if I had the problem (I didn't, thank goodness) I would consider a separately watermarked copy of the file for each..perhaps the first five letters of the student's last name. That, while it may be possible to alter, would require more effort than it is worth. Working with watermarked images is the norm when using many of the current Photoshop books in which access to images is provided so that the reader may replicate the examples shown in the book.  (The more unusual the type face, the better.) You may save the watermark on a layer to be applied to each assigned image and then saved for the student as a jpg, so you have to create the watermark only once for each student and drag the layer to new assigned images. Good luck.