That is a bummer. No one - parents, grandparents, teachers, uncle, and aunts - has the right to correct or punish someone for using the opposite hand.
I am naturally left-handed. When I was learning to feed myself, I would use the left hand. My grandmother would take away the fork or spoon, slap my hand hard, then replace the filled implement in my right hand and force me to eat like that.I would take one bite to oblige her, but the next bit would be with my left hand.
My mom was watching.Finally, she stepped in. "I think she's going to be a left-hander."
Grandma objected. "We don't have any in our family!"
"Well," Mom reasoned, "She'll be the first."
That was several decades ago. I learned to write left-handed in school, and nobody said one word. I used crayons, paints and markers with my left hand. Nothing.I reach for things instinctively now with my left hand first . Those right-hand-only desks were a detriment to my writing anything.
There is nothing "wrong" about being a left-handed person. We are just "different handed"is all. I'm sorry for what you went through at school.