It's a complex (and dark!) little movie. I really do like it, and the coda does move us back to our normal idols, complete with MVP Kotori punchline.
My reconstruction of the movie plot, if anyone is still confused:
Yukiho and Haruka were friends, but when Haruka was being bullied, Yukiho didn't or wasn't able to stand up for her, and when the bullies forced the issue, Haruka committed suicide. Yukiho transfers schools.
Skipping back a bit, Iori transferred into Makoto's school after a scandal, put together a girl gang including Miki and Hibiki, and bullied their teacher, Azuza (Makoto's mom), into a psychotic break. They then turned on Makoto, until Makoto snapped, put Iori in the hospital, and became a bully "himself" — "his" gang includes Ami and Mami. Iori transfers out again. Miki takes over her gang, and mildly bullies the replacement teacher, the Producer, mostly by coming on to him.
Chihaya caught Makoto's gang's ire by defending an underclassman, and became their regular target.
That's when Yukiho transfers in, apparently staying with Takane (maybe also a teacher?). Her first day, she sees Makoto and the twins beating up Chihaya and befriends Yayoi, then discovers that Makoto was her childhood friend, and about "his" mom's status. Makoto says "he" will protect Yukiho from other bullies.
The teachers, Ritsuko and Producer, complain that they're helpless.
Yukiho contemplates the situation at a bridge, and Chihaya talks with her, which prompts flashbacks to Haruka's death. Someone turns Makoto into the school authorities for beating up Chihaya; Makoto's gang beats up Chihaya again until Yukiho reveals that she turned "him" in and turns "his" own words back on him. Later, Yukiho takes Chihaya to Makoto's house to try to start the healing process.
(Also, I didn't realize until this read that Chihaya was also playing a boy, but she's wearing the male uniform just like Makoto and the twins.)
