How do you think Umbridge would react if during her reign as High Inquisiter and Headmistress, an epidemic of a diease of a somewhat sensitive nature took hold of the student body? It turns out to be just a elaborate prank by the Weasley twins, but all the same, it could mean her job and position, as well as her image in the media.
What would her reaction would be once the epidemic first took hold? What would she be willing to do to keep the outside public from finding out? What would her reactin be upon finding out the entire event was a stunt (keep in mind, it wasn't the student's fault, although she may not see it that way. It was purely the Weasley twins)?