When I was 1st grade in elementary school, there was a small bullying.
One day, one transfer student joined in my class. She has lived in America for some years, so she was so-called returnee student from abroad.
Now, if I meet a returnee student, I try to understand his/her background and I'd like to know what kind of experiences he/she had in the life.
However, it was elementary school age. We were just kids. We didn't try to understand her. In addition, almost all the classmates couldn't accept her who is a returnee student and didn't grow up same as everyone.
One day, in one class, we studied by ourselves because the teacher was busy to do something. Untill the teacher came back to our class, we were told to study Chinese character, kanji. We had one word which was difficult to read for everyone(The word was「文字」. We said we could read this「ぶんじ」though it was「もじ」). Then, she, the transfer student could read the word and she taught us how to read it. Actually, she was a clever girl and her answer was right, but we didn't agree her. We said "That's strange." or "I can't believe you." No one helped her. I feel sorry for her now.
I think children are kind of conservative. They can't accept things or people that are different from themselves.
Bullying is so spread because it became habitual, frequent thing maybe. If others do a thing and it is thought habitual, people don't get to feel it's bad thing, and it spreads around at once. To riduce bullying, adults can help. The lecture of young person who experienced bullying at school days may affect positively to student's thinking about bullying.