I have used Wallis Sanded paper personally for drawing with dry
pastels. If it is for your class though, you might want to try mid-
grey fine sandpaper from your local hardware store. I have used it in
the past and it works great. It is not archival so I don't know if
that needs to be taken into account or not.
Thanks for all the great tips on grading.
I appreciate all of the comments and am now better equipped to give
them a fairer grade at the midterm.