I just wanted to add that, as a parent who's daughter was recently accused of something at school which she vehemently denies, I do view these situations from both sides of the fence. It would appear from my daughter's point of view that she was guilty by association with the culprits.
Having discussed the matter with her teacher told my daughter that I believed her but, I allowed the school to deal with her as they saw fit. I reasoned that if she really was telling the truth it would discourage her from hanging out with the group of so-called mates who were happy for her to take the rap with them and encourage her to steer clear of any trouble in future. By the same token, if she wasn't telling the truth she got no payoff for lying to me. Despite what many parents believe, 99.9% of children tell fibs, especially when they think they are in trouble. I am under no illusions that mine would be any different.
I can only hope that I handled the situation right. In my daughter's case though, it didn't involve suspension so I'm not comparing her situation to your son's. I'm just attempting to illustrate that I'm not automatically biased towards school staff just because I am one. I am also a parent who faces life on the other side of the fence too.
I wish you luck in getting to the bottom of your son's situation.