Did the girl who died 26 years ago exist in the same way as the extras?
It's a good question.

I think the class just see her ghost. In fact, the ghost of Misaki felt herself loved and so she stayed all the schoolar year. But this fact created a curse, death people (extras) came to the class the next years to feel like Misaki and when the natural numerical balance of student broke, people started to dying. 

That's what I understood when I saw the anime.