You’ve said girls resent CP. That’s simply untrue. Some girls resent CP, but so do some boys. Some girls (and some boys) resent other punishments such as lines or detentions – which is why, whenever I got either at school, I simply didn’t do them knowing full well I’d get the slipper instead. Most, if not all, the girls I associated with had the same attitude.
A discriminatory approach also ignores the problems caused by treating one group more favorably than another. You recently posted a link to an article stating that children were being illegally excluded from school. In some cases, boys were being excluded because they refused to obey unlawful, discriminatory rules on hair length. It’s likely that some of the others, who may have been excluded for misbehavior, misbehaved because they resented being discriminated against.