This story is definitely not for kids.
When a new student transfers into class 3, he finds a strange place, where one student is completely ignored, to the point that they might as well not exist at all. He finds himself wondering if the one might be a ghost, or something only he can see.
Then it begins, a calamity that has plagued the students of class 3 for twenty-five years. A curse where one person connected to the class dies every month in horrific, twisted accidents.
The curse began when a popular student passed away unexpectedly at the start of the school year. The students, and even the teachers decided to deal with the loss by simply pretending the student was still there, sitting at their desk. Their actions brought the spirit back, and ever since then, Class 3 has had an extra student, one that was supposed to be dead.
The dead class member doesn't know they're dead, or that they're the catalyst for the horrible accidents which plague the living members. At the end of the school year, they simply cease to be.
The new class deals with the problem by completely ostracizing one member, treating them as if they don't exist, and making the number of students in the class equal to what it's supposed to be.
The new student breaks the rules, talking to the ostracized member, letting the curse loose among the class. Then the rest of the class bands together to try to find ways to stop it before they all end up on the wrong side of life and death.
There's two main parts to this horror-mystery. The first half being the truth about why the class is cursed and whether the ostracized student is real or not.
The second part is the search for a way to stop the curse, and who/how many of them are going to die before they discover which of their classmates is the extra.
The tension builds up throughout the short run (only 12 episodes), culminating in a 'Final Destination' series of events and a mass madness which overtakes the students.
It's definitely worth watching as a horror fan, even if the first half is a bit slow at times. I'm a little late to the party on this one. The anime came out in 2012 and was based on a comic. A live-action movie was also made in 2014, but it's only available in the original Japanese, and I don't know that a dubbed or subtitled version of that exists.
Still, an excellent little horror series. Worth the time if you can catch it on Hulu or Netflix.