[TinyRedLeaf]

Originally Posted by Kanon

Spoiler:

Another was already present in class before Kouichi transfered in, so they should have already been one desk short at the beginning of the year as usual. And yet they weren't, and in the process were fooled into thinking this would be a safe year. What could this mean? That the extra person is not a student, but a teacher. That leaves us with Chibiki and Mikami as suspects. Depending on whether Mei included Chibiki in her answer even though he had already left, he could be ruled out (that also rules Reiko out...). I think it's more likely to be Mikami, who was the homeroom teacher two years ago.

Spoiler for classroom arrangements:

Actually, going by the class roster, they would have started the year with 29 students had it not been for Kouichi's late transfer. I understood 29 to be the "magic number" all along. As long as they had an empty desk in a classroom for 30 students, Class 3-3 could survive the year intact, blithely unaware of "another" existence.

Once Kouichi filled the empty desk, the class was forced to entertain the possibility of "another" among them who shouldn't be there. Hence the elaborate need to make one student "non-existent" when it was full, to take the number back down to 29.

How the previous students stumbled upon this stop-gap solution was never explained. It could have happened by accident. It seems that if you were to roster exactly 30 students for Class 3-3 the year before, when the year starts, you'd somehow end up with 31. Disaster!

So, to prevent that from ever happening again, they decided to start with just 29, leaving one desk always empty.

It was basically meant to be a confidence trick. If you start a year mysteriously with 31 students instead of 30, you right away become aware of "another" who shouldn't be there. So, they got around that by starting with 29. But once 29 becomes 30, it's the same problem again. Someone was added, when he or she shouldn't have been.