Theres no justice at my school,how about at yours?

I got a detention today for something I didn't even say. The reason says "disrespectful remarks about a teacher." The person who actually said the disrespectful remarks was another kid who no longer goes to our school anymore (he left today). I tried to fight the detention by talking to my counselor but she didn't believe me and neither does the teacher who gave me the detention. I have no say in this at all. I told my mom and all she said was theres nothing I can do, the teachers always going to be right. Is your school a dictatorship too?