I mean, it's shown that Kiritaka had his own sense of justice to the point he stepped in to help save the other kid from bullying, and willingly suffered for it so I'd definitely not call him a wuss
That reminds me of why I endured my share of bullying because I didn't want my parents to know or worry because I thought I'll get in trouble if I get dention or suspension if I fought back and was caught.
Because we all know as kids that telling the teacher is not going to solve anything in a public school system. The teachers can't do anything aside from informing the parents and that could easily not work or make things worse for you. In which case you end up either have to keep informing the teacher about it making everyone looks bad (because it implies they weren't effective) or you just accept it and give up.
However, what I will tell my kid is, if anybody ever tries to pull anything on you, you have my permission to grab a chair and smash him over the head and send him to the hospital. As long as I don't get called to hear you were the one starting shit, I can't wait to stroll into the school to hear what the teacher and the other parent have to say for themselves.