I mean the story is fairly understandable even without the letters the description talk about. There is no bullies, all the threats was made by the class rep to make Meari stay at home so she can bring her the papers and see her all the time. Meari seem to know that but she is acting like she didn't know about that. Simply put, Eri is a light yandere who want Meari for her alone even if it mean make her think she is bullied and make her stay at home and Meari is ok with that but don't say the truth because is she does, she think Eri will probably think that she hate her and stop coming and they won't see each other like that anymore.
It's a twisted love and their relation will probably need to had one of them saying the truth to the other to make it grow or maybe they will probably going out with each other and never said confess to the other about that.