Great film, but the kid did it without a doubt! Whilst the film had flaws in terms of predictability in both plot and the age old 'Liberals are good', 'Conservatives are bad' tropes, it's still a riveting and compelling watch.
However, watching it in 2020 it occurs to me the film is about something more than how prejudice can affect criminal cases, and more about how people can convince themselves of just about anything.
The only possibility that the boy was innocent was that someone else happened to find his knife and then proceeded directly to the boy's home to kill the boy's father. It would have to be someone the father knew intimately as he was in the house! This person would also need to be exactly the same height as the defendant.
It's beyond reasonable doubt that it was the kid who killed his own father. There is no other rational explanation...and so the film praised universally for being about prejudice in the judicial system is ultimately, really, about something else entirely.