Ep9 who was the killer??

so we have 3 possibilities: the teacher, the boyfriend and the girl 

I read a few comments about the ep, most people suspected the girl Kaew bc she was the one that was angry and so scared, some said she swore to kill him, but I don't think that's the reason behind the murder, if she was the killer she probably killed him out of fear, remember she heard noises and went to check, she was not in the right state of mind out of fear so when Koh (the boy who got murdered) tried to find her, she may have mistook him for the murderer and hit him. The lights only turned back on when they all went out which means she probably didn't see Koh at all and when she did release it was too late. 

The teacher may have been selfish and scared of Koh but he's a coward and even though he sacrificed some students, I don't think he would ever actually murder anyone.

I do suspect the boyfriend though, jealousy seems like a stupid motive but he seemed kinda like that type of boyfriend, I mean he even let the kid be thirsty to save water for his girl. Idk sth about him seemed off and dangerous so I doubt him too. And unlike Kaew I believe he did it on purpose and that was his plan when he got out to "pee", who would go to the bathroom with a SK around!! SUSPICIOUS!

I want to know what you guys think?

 I believe the teacher is more suspicious bcs he might be afraid of Koh. Towards the end of the episode, Nanno asks Tang (the teacher's daughter) to retrieve the phone that fell into the ditch. However, Tang lied and said that the ditch was too deep. The real reason for her lie was because she didn't want her father to get caught due to the evidence on the phone. Nanno herself mentioned that Tang's behavior was similar to her father's. From Nanno's words, I understood that they both had the same attitude, which is "deleting evidence for personal gain." What I mean is that the phone was strong evidence as it recorded everything that happened, but Tang claimed she couldn't reach it because it was too deep. She told this lie to protect her father from the authorities. Similarly, like Tang, it seems likely that the teacher, who is Tang's father, intentionally killed Koh so that he wouldn't report it to the authorities. 

That's basically my opinion.