I had this problem happen to me when I first got back into diving after years of a break. After a few dives that were perfectly fine, I went back into the water and just as you said, it hit me around 5 feet and below. Right behind/above my left eye was a horrible pain, I has to ascend to the surface to relieve it. I was very worried and email the DAN Emergency Hotline. They gave me a prompt response saying I should call a ENT doctor to get checked out. I did so, and they just said I have HORRIBLE allergies and the inside of my nostrills were the brightest red/inflamed they have seen. Now the allergies are slowly going away, I have had no problems and dive roughly every week or two. I have not taken any medication nor was perscribed anything, they just said over the counter allergy medicine might work. Hope this helps a bit.
I did notice however, that when this did happen to me, I would get out of the water for about 45 minutes and was able to go back in with no pain.. hopefully your students can do this as well.