Currently Airing Anime

I think that right now it's possible that the "Curse" is sentient. It didn't do anything the whole day, until they started talking about how to fight it, it then suddenly murders someone.

Interestingly, the only 2 survivors (that we've seen) of the incident 15 years ago both have memory blanks and fits of amnesia, and possibly something even more serious (judging by the actions of Reiko in earlier episodes). Perhaps ending the incident early causes it's own problems.

 Spoiler

All the deaths so far (at least student/teacher) have happened near the main character (forgot his name, i think it's keichi) and the heart attack death happened right after the guy was going to talk about the rules (and therefore break them)

As for the amnesia thing, I think whatever prevented the calamity definitely caused them to have amnesia. Some other-worldly source. Maybe telling other people what happened would get them killed so it's a safety measure. I can definitely see the anime adding some huge plot twist at the end of the series though that would create a "mind = blow" situation