Another/WMG

Only descendants of class 3 from 26 years ago can see Misaki.
That explains why only Sakakibara can see her out of the whole class: He's the son of someone from the original "cursed" class.

This may be eventually made obvious; but hey, I'm only assuming from watching episode 3 of the anime.
 * Jossed. The manga is being translated by several scanlation groups. It's rather deep into the story, and there it's been revealed that Mei  This may seem cruel, but the reason for this is very understandable:   in class 3. This   But everyone still knows that   The presence of   The only way to counteract this is to

Misaki manipulated Sakuragi into her accident because she saw Misaki.
It's not like it doesn't seem possible with the tone of this series. On top of that, Sakuragi may have been able to see her because her mother too was from class 3.
 * Alternately with the info in the first theory on the page

People are going to die in the order they appear in the Ending sequence.

 * Jossed
 * Alternate theory: Those that do die won't necessarily be shown in the ED, but if they do appear they will die in the order they're shown.
 * If that's true,
 * Double-Jossed:

The ED and OP are going to be Evolving Credits

 * Presuming the WMG above this one holds, this is what will happen to the OP and ED to match events in the story.

The myna bird's words are important.
They are the last words of the original Misaki, and Reiko is involved in their death some how, either through murder or by causing an accident (or in the latter case she at least she blames herself anyway). Her apparent drunkenness/insomnia are guilt with the bird acting a bit like Poe's tell tale heart.
 * It might also be that the bird is repeating Ritsuko, Kouichi's mother's, last words.
 * This seems unlikely, given how long ago Ritsuk's death was. Could the bird possibly have been alive then?
 * Depending on the breed some birds can live in excess of 25 years.
 * Third variation related to the above, it's the words of Kouichi's father when he was mourning Ritsuko.

The only way to break the curse is to discover who the dead student is and have them leave town.

 * Possibly Jossed as of Episode 10.

Mei can see the extra.
There's got to be something about her Doll-eye. Mei mentioned being near death once due to a tumor in her eye, which had to be removed... and in Episode seven, when she reassured that Koichi  Which gives a strong possibility that she's able to see who the extra is, or may already KNOW who the Extra is.

The Extra may in fact, be a ghost from 26 years ago whose possessing someone every year to MAKE the extra.
As in, someone, some poor schmuck (most likely directly related to the ghost) is being possessed and somehow directed into being a part of Class three, therefore, causing an unbalance each year.

The Extra may be subconsciously aware of the countermeasures taken to protect Class 3 and work to subvert them.
This is also assuming that Kouichi is in fact The Extra. With information given that even with the same countermeasures taken, the success rate is only about 50%. Kouichi ended up being an unexpected wildcard in the balance otherwise, and ended up triggering the calamity for that year. It could be that past years that failed were due to The Extra of that year either breaking the rules, or becoming the Forgotten and cracking under pressure to ensure people acknowledged them. It also doesn't help that Kouichi has had personal interaction with almost all of the victims so far, and present for or witness to their deaths in some way. Could be that The Extra instigates the events, and the closest ones to them for that year are the ones that died, creating more 'candidates' for the next cycle. In essence, the success of the plan relies on the will of the students in the class. For the class that had invited death in, the years it failed had students that unknowingly invited death into their lives though the acceptance of The Extra.

The people in the end of episode song background picture will live
Correct me if i'm wrong but so far none of the people that fade into the scene have been killed, however Misaki fades OUT of the picture, possibly meaning that she will be killed by the curse.
 * Dude, you're totally wrong.
 * My mistake then, i'm pretty damn bad with names and faces, carry on WMG.

The "Extra" is...
Kouichi Sakakibara Mei Misaki Ritsuko Sakakibara, Kouichi's Mother
 * The students who visit him question that they know him from somewhere,
 * This seems to be supported by episode 6:
 * Also seems to be supported by . In the   If this is true, then   combined with.
 * Also seems to be supported by . In the   If this is true, then   combined with.
 * The series seems to make effort to avoid the obvious answers as far as possible, so a double-switch might be a potential solution. She has a credible death-story as well (her eye-tumor at the age of four), and her strange relationship with her mother might be due to the fact that they haven't actually had a mother-child relationship in over a decade.
 * It's not been stated (to my knowledge) that the dead student actually died at that age so she could be there and a combination of her being under her maiden name and the curse's alterations of memories and records (i.e. the class photo would be changed) perhaps helped along by people's Weirdness Censor would hide her.


 * A consistent feature of the curse is that peoples' memories are altered to include the extra up until the point of either graduation or . This has held true for the current Class 3. One point on this matter that stands out is one occasion where Koichi is speaking to his father, who mentions having been to Yomiyama one year ago, conflicting with Koichi's memory that he'd never been there after his birth. His father backpedals to go along with this right after. There should be no reason to change such a recollection, unless perhaps Koichi had direct contact with the Extra during that time. Who did we see Koichi meeting in flashback in episode 10?.
 * Ultimately


 * He just didn't die like Teshigawara thought, and that's why.

The curse trying to kill the Extra is a Shinigami with a Death Note
The Shinigami Realm also suffers from the dead person masquerading as an alive one, so the King tasks one of the Shinigami to kill the Extra; the problem is the Extra's influence on memories alters its own death date thus making the Shinigami eyes useless, so he start randomly killing the students until he gets the right one. Hence why a lot of the deaths are so illogical; it's because the bored Shinigami is playing around with how many ways he can kill a person. didn't die from a natural heart attack, but because his written death was too impossible and so the Death Note simply killed him via its usual method. Once the Extra is dead the Shinigami goes back to his usual job until the next year when another one has to do it.

There will be a sequel set in modern day
It will involve a new class trying to stop the curse once and for all. Misaki or Kouichi might make a guest appearance as a teaching assistant or school staff. This first series would essentially be a big prologue to the sequel, which would explain why it's set in 1998 for no apparent reason.

The curse was started by a Witch.
A burakumin girl (the descendants of Japan's Buddhist equivalent to the Hindu "untouchable" class, who still face some discrimination) was a member of Class 3 back in at least 1946 (26 years before the curse's previous appearance). Her family was of course rather poor, and so she wished to be able to pass the entrance exam to Yomiyama North Middle School. The class had only 25 desks (just one short), and so she was forced to stand. On top of all that, postwar Japan suffered severe food shortages, and not only could her parents not afford to make her a lunch, she had no money to buy and her classmates refused to share theirs. Eventually her grief and loneliness at being from "the bottom caste" and literally "the odd girl out" consumed her.
 * The "extra" is either a familiar of hers, or perhaps just an entity created whole-cloth from her powers, assuming the form and personality of one of the class members' recently deceased relatives appearing every 26 years, in recognition of the 26 students in the girl's class. And the Higurashi-esque (ahem) Witch Hunt at the end of Episode 11 was just as planned by the Incubators, knowing the scared, suspicious students would likely suspect a quiet Strange Girl like Mei, who would also have possibly made a contract.