[Dengar]

It's my belief that it's something like, the dead one is something that shouldn't exist. This puts a massive strain on reality, which eventually starts to cause people to die. However, the deaths don't rectify the problem, so the problem persists, so people keep dying, until graduation, when the dead one finally disappears. OR until the dead one disappears prematurely.

Killing the extra is however, a very harsh thing to do, because they look and act like humans, and are themselves unaware, and are themselves not directly responsible for the deaths, but at the same time, there's one thing that's true: They are already dead, and they WILL disappear at graduation and it'd be as if they were never in that class. Killing the extra would merely speed up this process.

Of course, resorting to this unless you are 100% sure that you have the right one is borderline murder. Don't come with stupid theories like "She only has one eye because she came back wrong.". That's just dumb. "The deaths only started when you showed up." can still be coincidence and does not stand on its own.

Forgetting for the moment the supernatural hax ability to see the color of death, the best approach is for the entire class to work together on this and perform thorough investigation on one another. It's evident that not all inconsistencies are erased, such as the fact of a class having an assistant-homeroom teacher when there is no need for such a thing. Granted, what with the perception filter, that's probably pretty hard to notice, but if like 30 or more people think it over very thoroughly there should be SOMEONE who can draw the correct conclusion, right?

It's also best to do this at the start of the year, when not everyone is so emotionally scarred by all the death and destruction. This goes without saying.

Perhaps, if done consistently and effectively over the course of the years, maybe the dead will realise that they aren't welcome anymore and stop popping up.