All the explanations you need here...
- Chris is the one who brings up the 4.0 idea to Tim early one. He says he heard it on MTV. (It could have also been Rand who thought of the idea all along.)
- Chris pretends not to wanna go through with it (killing Rand) after Tim makes up his mind on it.
- Chris knew that Tim wouldn't actually throw Rand over the cliff in the beginning, because Chris was actually in a master plan with Rand all along. Rand told Chris about the plan he had with Tim to double cross Chris. Tim obviously does not know about this 'till the end of the film.
- Rand had a plan with both Chris and Tim, the master plan being that Tim would die. But, if Chris were to have died at the end, (due to maybe Tim pushing Chris off the ledge) Rand would still have been in the clear in terms of being a suspect and getting his 4.0, which I believe he does not take at the end.
(It seems to not be what he wanted all along, but rather to make a twisted plot among Emma and Chris and/or to have them look like suspects in the end just for his sheer enjoyment or "drama".)
- Emma is working with Tim and Rand and planning on double crossing Chris. (Which is maybe why Rand knows about Chris' sexual problems.) Rand knows that Natalie dying is part of the plan with Emma, which is what his suicide also is intended for.
- Emma's plan involves making Natalie think she's pregnant which leads her into problems with Rand. Emma knew that along with Natalie's "pregnancy", that once Rand "killed himself", she would end up killing herself which would give Emma the 4.0 since they were roommates.
- Rand is purposely a jerk to Natalie.
- Tim is trying to frame Chris to "push him over the edge" so to speak, not knowing that Chris is in on what Rand and him are trying to do.
ONLY PROBLEMS:
- Why don't Rand and Chris just kill Tim in the beginning?
(Probably because that would interfere with Emma's plan.)
- Is Emma really with Tim the whole time, or only towards the end because Tim has convinced her of his plan with Rand?
(Probably the whole time, I think Emma and Tim knew that the school counselor lady was listening to them talk about Chris at the cafe or whatever.)
- What was the point of recording Tim on tape?