In your opinion*
Pure speculation.
How can you be satisfied just believing that every single person is simply mistaken? You're smarter than that.
The school teacher confirmed Luke had a parka because staff joked about Luke resembling a hooded monk with it on at school. It's these wee things that stick in your memory. Before the murder. This teacher had left the school before June 30th. Why would his brain conjure up this false memory and why would an educated man involve himself in a murder trial without being 100% sure?
As I previously mentioned is far too easy an explanation to just rubbish every every single witness as easily led/dim witted etc and one that I wouldn't be comfortable just accepting so readily like many of your followers. Just like you rubbished those quotes that appear nowhere else online as song lyrics etc.
Luke's friends all confirmed he had a parka, it would be pretty easy to remember a jacket a friend owned without being confused by a newspaper photograph of a person you'd seen countless times before. No doubt they'd have numerous occasions/locations in their memory of him wearing it. I can picture in my mind what jackets my friends own right now.
There isn't a photograph of him wearing a parka before the murder? I don't believe any of us have seen a picture of Luke before the murder fullstop. And why would there be pictures of him doing such mundane things as being at school or hanging around with mates? I was 14 at the same time as Luke in Edinburgh and can confirm we weren't bringing cameras to school and taking pictures. (This was before camera phones yet - Nokia 3310s etc were considered top of the range)
Luke's friends who actually knew him and witnessed his behaviour unlike you also find it easy to accept that he's guilty. Why is that? Why does Shane find it perfectly possible for his younger brother to be guilty? Does his dad believe him to be innocent?
How many teachers went to court. Did more than one of the staff that joked in staff room confirm this?
I'm sure Luke's friends did say this, there's also a lot who don't think this. And even more sow as time passes. It easy to say things to try and distance yourself from something so horrific, especially at the time. But after all, as you say, they were his friends and you would have to presume were into mostly the same things,went to the same parties, smoked pot together, listened to much the same music. Except they weren't accused of murder. If the shoe was on the other foot , how many of his friends could luke have said exactly the same things about.