In this situation, given that we do not know all the details of the case, my decision would be to punish both boys, but give a more severe sentence to the one who hit the bat, as he caused severe harm to the bully’s health. As a result, the bullies would be punished for bullying the boy, and the boy for lashing out at them, his revenge was far worse than any bully action.
(I don’t know how it is in Canada, but in Russia for bullying at school, parents must pay a fine of $ 10 and no more punishment follows. And for assault and bodily harm, criminal liability comes into play and the appropriate punishment is imprisonment for a period of 5-8 years.)