Help me comprehend Death Bell? *contains spoilers I know about*?

[Sophia]

I watched the Korean movie Death Bell and was slightly taken aback. The movie was good, like a play on Saw, but with students who are solving puzzles to save the other students instead of the ones being tortured solving it themselves. So I know the teacher is Jiwon's killer, the ranking was from when Jiwon was pushed to 6th. That guard is Jiwon's father and never intended to hurt Ina, yet he has murdered someone and also he murdered the male teacher (Mr.Kim?) << (his name was different in the listing on wikipedia, so what is the male teacher's name?). then all of a sudden, I heard that the mother was the one who killed the ranking students, or something? Also, because the quality wasn't good, and it was dark, can someone tell me how each student died after the first one was drowned? I saw their bodies, but it was too blurry and dark to see how they were killed. So what I need to know is: Is the mother of Jiwon the killer of the students, that male teacher murdered Jiwon and yet he genuinely didn't plot or help in the murders of the other students and was just as scared as they were, the guard/Jiwon's father is the one who finally got revenge, but did he help plot the student murders? confusing!!!