[Kanon]

Originally Posted by TinyRedLeaf

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.

No, there is never an empty desk. That's precisely how they can tell there is an extra person: they are missing a desk since one student was never accounted for.

And there's no magic number. Take a look at the list from 1996, it presently indicates there were 30 students. That is after the extra one, Asakura Mami was removed. That means they were 31 (30+Mami) that year.