No, it didn’t happen overnight – it took a couple of years. But it did happen. There was a massive deterioration in overall behaviour in schools in the UK in the years following abolition, and the deterioration was far more dramatic among boys than girls.
Previously, boys were disciplined (in all senses) and girls were not. Overall discipline will reduce if you leave boys undisciplined too. The boys possibly thought they had a bit of “catching up” to do too. Misbehaviour by girls was (and is) often overlooked so that would account for some of the difference between the sexes.