I'd tend to agree that this may be an issue with the fixer, although I've never had this happen myself. I've had problems with fogged paper, old expired paper and exhausted developer, and this doesn't look like any of those.
If your students are using a water stop bath, or exhausted stop, this will transfer more developer over to your fixer and exhaust it more quickly. Since paper substrates are more absorbent than a film base, I think it's more critical to use a stop bath with prints than when developing film.
I'd use fresh chemicals and re-test with the same paper. If it keeps happening, then look at the paper as a possible cause.