You know, M's summer school program used to do this to me. I'd get to the school, and they'd race up to me to tell me what BS stuff he had pulled that day. That way I was sure to be good and PO'd at him the moment I got him into the car and we'd have a fight. Every stinking day.
In hindsight, I should have directed those people to make a list or set aside a time to talk privately with me without getting me all riled up and mad at him for something he didn't do to me. They need to deal with it or tell us he can't go there.
Good idea sometimes they just are full of it. Collecting a bunch of stuff to get you riled. The way they behaved it makes me think they do not like your kid and want you not to like him either. The martial arts folks did this sort of thing to me as an adult made my parents jump on me so I quit martial arts and told the director what she could do with my black belt I'm afraid she didn't take it very well.
They were threatening to throw me out so I just left and they hassled me about coming back afterwards as if they'd forgotten that they'd threatened me and set my parents on me like you described so I decided to take care of the problem they had with me perminantly by ditching the martial artists. I figure they just wanted money and none of it was real the encouragement or the stuff they preached so I threw it out. Why pay to be frustrated or verbally abused it makes no sense? People get upset and want to make other's lives difficult because they can and it's human nature. You made some mistakes everyone has on this earth. I'm glad you have enough courage to admit it yet that happened then this is now and you have to find the strength to meet the coming challenges rather than worry about might have beens or should have beens. What matters is you learned from it and took responsibility for your problems. That takes more courage than most people I've met have. Remember hope and keep the faith.