I'm a bit confused: didn't they say that the class started with the right number of students before Koichi enrolled? Then how could the calamity have started in April (before Koichi enrolled) like Mei suggested when the class started out with the correct number of students? Would it had only started after Koichi enrolled like the other students believe? Unless there was already an extra student and people just couldn't count. 
- nvm
I first thought Akazawa blaming Mei was bad character on her part but, after hearing Mei said she knows she has the ability to differentiate the extra student all along, then I think Akazawa's blame on Mei is partly correct. Mei had the ability to help others but withheld it because of her own fear and denial. Although I understand that prior to the tape, she probably felt it was useless since she didn't know what to do even after identifying the other but she could have helped Akazawa in some way by revealing the information of her ability.
But I guess Mei decides to reveal her skill now since they know how to possibly stop the curse.
My main suspicions are on the assistant homeroom teacher (I guess she's now the homeroom teacher) and on that glasses girl student who was with them on the beach trip. She's often around the main characters (walked by Koichi in this ep), shown in the ED, but we hardly see any interaction of the main characters with her or even her with us viewers. Why is she on the ED when she hardly talk and we hardly see her? Also, she gave Koichi odd glances in the very first episode but then became background after that. Suspicious!
Edit: Also, why was the name of extra in the 1993 class got deleted? It's not like it is the same dead student that comes back. Knowing who that year's extra wouldn't make any difference in the present year.