My teacher thought they knew my name better than me

This was years ago now when i was in secondary school (around 15 years old). We used to have form in the morning where the teacher would take the register and let you know about anything going on in the school before you went to your other classes. My form tutor was a miserable old woman that was a renowned arsehole. There were several stories i could tell about her but this one is the only time i got the better of her.

My parents named me a shortened version of posh sounding name, for the sake of the story lets say they called me Alex which is short for Alexander. When ever this woman would call my name she would always use Alexander. I brought up to her that it was not my name multiple times and asked her to please call me Alex as thats what my parents called me.

She would always get angry and tell me "Dont be stupid, no one is named Alex. Your name is Alexander, Alex is just what you want to be called." No matter how much I insisted she refused. At one point she gave me a detention for asking her to call me my correct name.

The school called to let my parents know i had been given a detention for arguing with ny teacher. When I told my parents I was supposed to have a detention for asking my teacher to call me the right name, they were not happy. So they gave me a trump card to use against her: my birth certificate.

The next day when she called my name, I once again told her that wasnt my name. She theatened me with another dettention so I pulled out the birth certificate, put it down on her desk and said "my birth certificate says my name is Alex so thats what you will call me thanks"

The look on her face was priceless, and she started calling me my actuall name for the rest of the time i was in her class.