I believe accidental strike with knife, they decide on over-kill to make it look like someone intentionally came in and killed her, then all 3 run out. This could have all happened within a very short time period.
They could have even gotten back in time to open up Naruto at 9:26 for alibi purposes.
They could not turn on their phones to call someone for alibi purposes, b/c they had already turned their phones off for the prank. So their phones were off. If they turned them on, that would be obvious give-away - phone turned off at 8:42, then turned on again at 9:26 or whenever, that would be very obvious give-away as when the murder happened, and also that Amanda and RS phones were off during the time of the murder would point straight to them.
They could not see anyone for alibi purposes, b/c they decided at the cottage afterwards, when they were coming up with their plan for what to do - that their alibi would be that they stayed in all night and their alibi would be each other. Besides, they did not have time to meet anyone for an alibi, becaues first they had to clean themselves up, and then they had to go and take care of their business at the cottage (staging).