I'm not sure my answer will be enlightening...
I learned out of sheer necessity after I had poked so many holes in the tip of my finger that I had to wait a week to do any more quilting! The first half dozen thimbles I tried didn't feel right (too small, too large, dimples on top not deep enough to "hold" the needle). As I was about to despair I found a cheapo plastic one that fit perfectly, and I could finally stop concentrating on keeping it on my finger and focus on my stitches. That worked for about a year, then the plastic wore through (unbeknownst to me) and I put a really deep hole in my finger, and was right back where I started...I now have a metal thimble wrapped on the outside with several layers of masking tape to stop it wearing blisters into the neighboring fingers.
I also learned to thumb quilt, with a thumb thimble, for when my hands got too sore from regular quilting.
Synopsis: maybe provide a really wide range of thimbles for your students to try (metal, plastic, leather, all different sizes) and have them focus on what they feel is the issue with any given thimble (is it sliding around? is it pinching? is it rubbing the inside of the fingers beside your thimble finger?) For me, if the thimble isn't so comfortable that I forget I'm wearing it, it just isn't going to work.
Alison