Luke Mitchell had no alibi after leaving school. His claim to have gone straight home was shown to have been a lie by his own brother who was in the house. Consequently there is a period of almost 2 hours unaccounted for after he left school alone.
He was seen by two independent witnesses standing just yards from the murder scene staring into space and just a matter of minutes after Jodi's murder. Nobody stands at that particular spot for any reason other than they are up to some mischief. Mitchell was unlucky, he was caught as he was about to clamber over a gate which led to a small wood and access to his house avoiding the main road. Remember the neighbours dogs who followed his trail home?
This wasn't the first time he had held his favourite hunting knife at a girls throat was it?