Huh? So what should the girl be taught to do?
Walk away and take it "like a girl". Seriously if a weaker kid hits a stronger kid, the latter should not retaliate -- regardless of gender.
For his own protection, he needed to be taught and understand that if he hit a female, the cultural backlash would be swift and damning- whether she was two inches taller than him at the age of thirteen, or not- doesn't matter.
And if a boy has been raised, or conditioned to view this as a definite taboo in terms of his behavior as a 'man' from boyhood, he's much more likely to understand that when he reaches the age of fifteen and has on average twenty pounds and more muscle than most females his age, he'll be more likely to adhere to this admonition and keep himself safe from maybe even inadvertently hurting someone smaller and weaker than himself while never being the agent of abuse and harm to any of the women in his life.
Society is to blame for this evil anti-boy sexism.
Men/boys should not be raised to treat little girls much differently from little boys. Chivalry and gentleness towards females should be reserved for adult women.
Have you ever thought of the consequences of such sexism-against-little boys? How it can actually be counter-productive to the protection-of-little-girls?
Here is an example of how.
Let's say a little boy notices men treating little girls better than little boys. He feels extreme indignation because -- if the adults in charge are men -- girls have a free pass to do pretty much whatever they want to that poor helpless boy. If the boy hits back, then men admonish him for "hitting a girl". If the boy reports the bullying, the men admonish him for being weak enough to be "beat up by a girl". This no-win situation can drive a child crazy. It can make the little boy hate men and the little girls they defer to. What if the boy grows up with such repressed rage against men, society, and little girls? There is a high-risk that he will become depressed and contemplate suicide. Realizing that he might end his life, then he will figure he has nothing to lose by retaliating against society. In his retaliation against the macho male culture, he might end up heinously-abusing and killing many little girls before taking his own life. He might also end up injuring/killing even more girls in the process of committing suicide.
Sorry if this is disturbing but it's reality. Discrimination against one group of children can indirectly lead to danger to the other group of children.
You all may want to ask, if the boy grows up hating men for treating girls better than him, then why does he target little girls to "get even" with society? Why doesn't he limit his physical attacks to macho men?
Here is why. The little boy grows up thinking that the only way to punish the men who ruined his early-childhood is to hurt little girls. Is this fair or right? Absolutely not. However, this is the psychological state in which the little boy grows up with.
Don't get me wrong. I'm no fan of the "abuse excuse". However, if a little boy is abused by men who favor little girls, he will likely develop the belief that the only way to give himself justice against those evil men is to abuse little girls.
At the end of the day, anyone being mistreated by anyone is wrong and [ideally] should never happen. Sadly, society isn't the perfect society we all wish it would be.