Do you believe you should always believe your kids no matter what

I don't just mean in SA situations. I mean in everything. I have always told my kids I will always be on their side and I will always believe them. I was bullied by teachers and kids in school and my parents never believed me.

My kids are 7, 12, 14, and 17. My youngest is a girl. All the others are boys. My 17 year old got suspended the rest of the year for heating up another boy who was a freshman. His parents are trying to press charges because his head was slammed to the ground and they had to get stitches. The kid, his parents, and other kids are saying it was a bullying situation and my son was saying slurs. My son said that didn't happen and the other boy hit him first. I know the fact that other kids are saying my son was the instigator should mean that they are telling the truth but nobody likes my kids at school and people don't like me here so why would they tell the truth if it means they can get him in trouble? The principal isn't suspending the other kid because he's believing the "witnesses" and said he watched the security footage but my son told me there's a blind spot for the cameras and that's where the kid hit him first.

I'm fighting this and taking legal actions and nobody is supporting me. My own mom is saying if other kids are saying he was being a bully he probably was. The same women who never believed me at school. She thinks 10+ kids aren't gonna lie but I think if those 10+ kids don't like you they will. My dad isn't taking a side on whether or not my son was bullying. He just said even if the kid hit him considering how tall my son is compared to the kid, slamming his head to the ground is the excessive force anyway. I think if you fuck around you find out. Do you support your kids and always believe them? Because I feel everyone is against my family.