No one has said it is worse for a boy to hit a girl or that it's 'less worse' to hit a boy or said that female violence is ok in any sense.
What's been said is that there is a power imbalance between boys and girls. This is why we have 'gender' in the first place, or sex role stereotypes, which is a hiarchy males use to oppress females by using our biology against us.
There is decades worth, if not more, of sociological studies on this and unlimited amount of proof for anyone who bothers to seek it out. Gender studies, used to be women's studies, is a university level subject precisely because of this. Part of our socialisation is mysogyny in women also and many are blind to the reality (I know I was when I was younger, despite university level study on it). This isn't a new or out there concept, and while many ignore it it doesn't make it less real or concrete.
Female to male violence, or female on female violence is every bit abhorant but it isn't entrenched in our system (seen world wide) of class oppression meaning males are allways the dominant class. Class oppression is quite different from personal oppression and understanding the difference and how they effect us, the latter psychological and the former effects how we are socialsed. Male pattern violence is termed that because of how much more common it is for males to be the ones being violent, and for women to be vulnerable to it because of being victimised because of our biology. That term is not used for no reason. That doesn't in anyway dismiss the reality of female violence. The sociologists call it this because societies teach males it's ok to dominate and use women, and to use their increased strength and our vulnerbility to pregnancy against us. That's why rape culture is talked about as being a male thing also. No one is saying that women never attack men or other women, what's meant is that it's a social norm for men to claim women are asking for it, to victim blame, to justify preying on vulnerable women, to cheer on males who are targeting drunk girls or laugh at rape jokes. On the other hand it's not social norms for women to laugh about sexually assaulting men, to aim for the drunkest guy on a night out to have sex with him, or to take part in predatory behaviour like Woolf whistling, ass slapping, sexualized comments, or using (our non existent class power) to get men to sleep with us to get promotions. Of course the odd women may partake in these behaviors and use personal power in that way but that's not the social norms and the social norms of women being socialised into being more nurturing, caring, putting ourselves last, giving way to what men want, appeasing men to avoid their aggression and so on don't lead to those odd women being violent to men or other women. Male rape culture, male socialised norms, absolutely lead to male pattern violence and that's the difference, that's what needs challenging in these situations.
The not all men or women do it too arguments drive me crazy because looking at socialogical patterns doesn't dismiss these facts but it does look at why males much more commonly prey on women than the other way round. It especially bugs me as I was sexually abused by woman as well as men as a child and I strongly resent my reality (that women can be predators too) being used in a way that completely ignores my reality because female offending tends to follow selective patterns and is something that happens despite social norms and our socialisation, where as male violence to women happens because of social norms and male socialisation. It's this socialisation that needs challenged in these situations.
Like I said a couple of times I'm not sold the best way to do this is to say to this boy don't hit but especially not girls but I do think the school need to address this. Socialastion begins at birth (again numerous studies done on this) and none of us are immune from it which is why we often can notice it in front of us (I was most definitely guilty of this for along time, it's pretty normal) and this boy like most will know the likely hood is this boy knew that a girl was less likely to hit back as all boys do.
I actually have my own little sociological 'experiment' on this in our home, twin DS &DD both struggling with low frustration tolerance, anxiety, social problems, anger problems, impulse control problems and so on. (Asd &adhd). As much as possible I have raised them trying to be aware of how I communicate sex role stereotypes to them, as is dh who does a much larger share of the 'wife work' than me, and is a respectful gentle man. Yet my naturally gentle and sweet ds has lashed out in ways DD never has. He also has gotten alot more help and recognition for his difficulties than DD despite her being much more severely (my area of work so I know first hand) yet she still doesn't act violently to others. Women for the most part pose risk to themselves due to our socialastion and men towards others (part of why women are ignored by my system, take up much lower % of funds or secure beds etc even though they attempt suicide three times more than men -its put down to attension seeking when it does not work- and have similar rates of serious mh problem like schizophrenia to males).
We live in a country that victim blames, that teaches women how not to get raped, when we need to be teaching males, starting with boys, not to be rapists, not to see women as objects, to understand and find a way to move past their socialastion and to take responsibility for a culture that causes male violence. This should absolutely start young.