If you treat a child unfairly you’re highly likely to cause resentment and do considerable harm. Even if you try to be fair, by your own definition, the child might not consider it so and will react accordingly. I know how I would have reacted if I had been treated unfairly at school – my behaviour would have seriously deteriorated.
When corporal punishment isn’t available, it can’t be used even if it’s the right thing for a child. It guarantees that some children will never get what they need.
That’s my argument against exempting girls from CP in schools where it’s used for boys. It not only guarantees some girls will never get what they need, it also introduces the problems caused by injustice.