Very strange. It almost sounds like a bad batch of film. Are you bulk loading, or using factory film?(posted 8785 days ago)Just for something entirely different -- has there been any work done on your plumbing recently that might have stirred up old gunk in the pipes? I can't figure out what the "sand" might be.
It does definitely sound like some kind of fogging problem. Either there is still some light in the room when you load the reels, or those several rolls were exposed to heat, chemicals or radiation of some kind that your others weren't.
Can you shoot another test roll from that batch and take it to the teacher's darkroom and try with his chemicals? That might be a check on where the problem is coming from.