Okay. Ji-Won was only murdered by one person (Mr. Kim). However, the students parents with the top 5 scores also had a hand in it by bribing Mr. Kim into giving their children the test answers. When Ji-Won placed 6th, she realized that Mr. Kim had been payed off and she told him to fix her score, or she would go to the Board of Education (which would pretty much get him fired for life). Mr. Kim was then put in a position where he would probably be fired no matter what. If he changed Ji-Won's grades, the PTA members would have him fired for not honoring the bribes they gave him...if he did nothing, Ji-Won would get him fired. He kills her in order to avoid these consequences. When he kills her, she has on her cell phone camera and it captures the scene we see at the end where Mr. Kim's key-chain is dangling as he strangles her. In addition, the "crazy" student Beom sees the whole murder take place (which is probably why he is crazy and seeing Ji-Won's ghost everywhere).

Ji-Won's father is the security guard in the movie. Ji-Won needs good grades in order to get a scholarship or she cannot go to college..her parents are in dire straights. Her father once ran an unsuccessful business and ended up leaving the family to try and protect them from his creditors. When Ji-Won dies, we see that it doesn't fix the situation because the creditors (possibly mobsters) are ransacking her house looking for items to loot. I'm assuming that he found her phone on one of his rounds at the school, and realized his daughter was murdered. You see a scene where she's lying dead with a bunch of pills shoved in her mouth. He realizes that there's a cover-up and deduces that it probably had to do with her rank. He teams up with his wife to enact revenge.

Ina's theory that the students are being killed by rank is only partially correct. They're being killed by the rank they had 2 years ago when Ji-Won was murdered. Ina had actually placed 5th on the most current test, not 6th like Ji-Won. The five students who were murdered were the 5 that placed before Ji-Won, not the 4 who placed before Ina. They're the same students..Ina just managed to beat one out, despite the cheating. This implies another pay-off has happened.

It's strongly implied that Ina is being possessed by Ji-Won's ghost. The fact that Ina is getting screwed over in the exams just like Ji-Won is probably what triggered it...plus, they were best friends before Ji-Won was murdered. Beom can see Ji-Won's ghost in Ina and it drives him over the edge. Why, exactly he returns to the school--I don't know. But that's why he flips out and kills the wannabe boyfriend.

After Ji-Won's mother is finally killed, Ji-Won's father is still not satisfied because he still doesn't know who the murderer is. That's why we have another lock-down with the parents. Ina gets in the way, so he ties her up. He never has any intention of killing her. During the lock-down, one of the PTA member's finally realizes that Mr. Kim has murdered Ji-Won. Mr. Kim flips out since he has been backed in a corner. The security guard finds out it was Mr. Kim and murders him. Revenge is finally complete. Ina is now #1 in the posting and the father has killed his daughter's killer.