Many children have been damaged by bullying in school, because some children have not learned how to defend themselves from their parents. They have not seen their parents stand up for themselves or they get bad advice such as: "Just let it go and turn the other cheek".
Children often turn their insecurities and anger that they get from their parents, sorrow and sadness to other children. When you are insecure or have a lot of anger, then some children will take that out on others. I believe it is OK to do that to others who can defend themselves and give back. That way frustrations get resolved here and now instead of being kept inside until it gets too much and ends in violence, and children learn how to deal with emotions.
But those who can not defend themselves from that kind of bullying, just get sad and isolate themselves. Their self confidence drop and with that their feeling of self worth. It is bad that these children have not learned to stand up for themselves, because healthy children get stronger from playful bullying. Sadly many people never teach their children how to deal with these situations. Additionally it is a problem for these children because playful teasing is also a vital learning experience in how to flirt between boys and girls, but if you have not learned how to stand up for yourself, you will think that is bullying as well.
I have taught many children how to defend themselves from bullying, and not least taught many adults how to stand up for themselves at the workplace or in their relations.
Many children get raised to be good little children that do not hurt or upset other people, and that make the defenseless in the jungle of growing up.
I had a 14 year old girl in therapy, who did not want to go to school. She was bullied by 3 boys that obviously liked her, and this was their way to get in contact with her, but she did not see it that way, because she never learned the difference between teasing and bullying. To her they were just mean.
I separated her on a chair and told her to imagine one of the boys in front of her. I asked her what he looked like and what is his name? She said he was scrawny and red haired, and not very strong, also a bit shy.
I told her to try to say to him: "You do not need to have lights on your bike in the evening with that hair of yours". She laughed out hard! I told her to imagine changing place with him and imagine what he would feel. She answered me that it felt good to say something to him that made him speechless. I then asked her, how she thought the other boys would react to what she said. They would probably laugh, she said.
We met for 3 times a week for some time, and each time she learned some new defenses against bullying, and now she is very confident and has many friends. She now enjoys the teasing by the boys, but do not accept vicious bullying.