To execute a plan with so many variables on 1 set day is impossible...
To people who say this film is "the best thriller/horror/whatever ever!!!!" and that Dodger's plan is "very possible/easy" to pull off.
To start with Dodger wanted to get revenge on the girl who was cheating with Mr Mat, so she killed her. - yes all possible. Planting the gun etc. perfectly easy to achieve.
Now that's where everything was possible.
But now to get to the point - With the variables apparent throughout the film in order for Dodger's plan to work there had to be no mistakes, and sadly that wouldn't be possible. Here are some of the plot holes I have noticed...
1) how could Dodger have being able to rely on Owen a) being anywhere near his laptop, and b) had his laptop on when she sent him the messages?
2) None of the kids knew that it was Dodger on the messages so surely they must have been freaked out? How could Dodger rely on one of them not breaking down and backing out?
3) How could Dodger have known that when Owen was attacked in the car he would have run towards the policeman? If he had gone the other way, he would have realised when he saw the knife, and they wouldn't have been punished, therefore not be grounded in the school, therefore not being able to pull off the whole thing.
4) When Tom went after Owen, he swung at him with his knife. The only thing stopping Owen from getting hit was dogging out of the way. What if Owen hadn't moved?
5) Dodger planted the gun in Mr Mats draw, middle draw. So lets get this straight the guy wouldn't have opened his main draw on his desk in the last few days? Whatever.
6) I forget which character, but one of them saw Mr Mat before he went into his office. How could Dodger have relied on Mr Mat getting into his office without somebody telling him about Owen? Also where was Mr Mat before he went to his office, why hadn't he seen anything?
7) So lets get this straight, Mr Mat didn't dump Dodger, yet he put his neck out to keep Owen, the guy who was getting off with Dodger, in the school! What if Dodger had been expelled?
8) How could Dodger have kept the plan to one day, at one time? Halloween. As I've said. ONE uncontrolable mistake and it would have been over.
There are many more reasons. Care to add to them? But the fact remains. It would be impossible to pull it off.