I got called a slut and groped at school

I live with my mom but see my dad on weekends. I have a step brother on my dad's side. He's older than me. He's 18, I'm 16 but he was held back and in a junior and I'm a sophomore. He's autistic and this group of kids were picking on him every day he went to the bus so I started walking up earlier to walk to my dad's house and walk to the bus stop with me. Me and him have honestly gotten really close. I consider him kind of my best friend but my mom says I'm not allowed to call him a friend or a brother and he's a step brother nothing more. I saw him in the nurses office in tears from a panic attack Wednesday. I asked what happened and he said he got pantsed during gym and someone took a picture. He's younger minded and like his underwear to have cartoons. I comforted him and called my dad. My dad picked him up and thanked me. At the bus stop this guy I hate was flonting it so I went up to him and grabbed his phone and deleted the picture. He pushed me. I got up and his sisters pushed me back and held me on the ground. The guy said "Slut" and grabbed both my boobs. I told my mom and she didn't care. She said he groped me because I'm too fast for my own age. My dad went up there and made that boy piss himself. I am not even lying. This kid is a senior and my dad is so scary that he scared that boy into pissing himself in front of his friends. He got detention. But how everyone is school is calling me a slut for no reason...