yes, the bare minimum, is not even about believing in him but about not jumping to conclusions without any fucking evidence.
yeah sure, "one mistake", the mistake being making him a target of bullying that resulted in his stuff getting stolen, not fixing things even tho she noticed immediately that she did something wrong, and even tho she had a lot of opportunities to fix things. Slapping him when he dared to try to prove his innocence, and never even apologizing to him (ofc she did apologize to the parents, so she KNOWS how to apologize and just chooses not to do it for him). Is it really emotional immaturity to cut off someone who wronged you this way and, once again, HAS YET TO APOLOGIZE? It sounds to me that you just like ppl getting abused.
The things that a teacher did and caused could leave scars for your whole life, given the MC's age at the time. The only reason this is mostly not the case for the MC is that he was already basically dead inside by that point. This teacher single-handedly destroyed a whole class. She was prepared to throw a little kid in the fire, but couldn't take the heat at any point. Even when she knew she was in the wrong, she kept trying to save her ass, like when he, after proving beyond any reasonable doubt that he was the victim, asked her to the most basic shit "You called my mom even though I didn't do anything, so at least do the same for them, because everything they did really happened" she STILL tried to save her ass and "I will properly listen to you" her way out instead of agreeing with his basic terms, and she was going to find her way out if she wasn't checkmated by the appearance of the vice principal.