Shizuka (the glasses girl) liked Azami (the other girl) since first year, because Azami asked her to be friends, after they got separated, Shizuka decided to plot to get them together, and by the looks of it, she probably said something to some random girls in Azami's classroom, then the girls started bullying Azami in her 2nd year.
Then Shizuka decided to forge her own bullying and have "coincidental" encounters with Azami ending up getting Azami's sympathy.
And when the 2 other girls got close to Azami, Shizuka decided to get rid of them (not literary lol) by saying they are together with the bullies in Azami's class.
That end is just the 2 girls who where really friends of Azami getting Shizuka red handed forging her bullying, but since Azami got there after they went to check, Azami thought the 2 girls are really together with the bullies and took Shizuka's side.