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Fridge Logic: How are the chavs and their parents going to get away with murdering Jenny, especially given that the Indian boy's parents are not among the party guests and are surely going to report him missing. In addition, Paige and Cooper's bodies are in the woods. Besides, I doubt some of the party guests are going to keep silent when the police interrogate them about the missing couple.
Plus, what could they tell the police that would cover everything? If they pretend they know nothing about Steve and Jenny, the police will want to know what happened to the dead kids in the woods, and there's a car and Steve's body in the woods. The adults will also now know the kids were lying before, which might convince one of them to speak up if they hadn't already. But if the kids stick to the "self-defence," story, then there's a fuel can with only Brett's fingerprints on it (which can be chucked or wiped, but that'll look suspicious in itself) and Steve's body and probably Jenny's really won't look like they were killed in self-defence. And the kids will have to construct a specific series of events which explained all the evidence, which would be tricky. It just doesn't seem plausible that they could get away with it.
They probably won't get away with it but they are dumb enough to think they can.