There are two types of bullying.
The first is not serious and mostly involves kids trash talking one another, and some physical pushing, shoving, and the occasional punch to the upper arm or the back of the shoulder. The worst might be the class jerk gets stuffed into a locker. No one gets hurt, but it teaches kids how to deal with idiots. To some degree it also enforces societal norms. When I was a kid, if a boy came to class in a dress, he would have been teased and bullied without mercy. He would have quickly realized dressing that way was not acceptable to his peers.
The second kind is what is not acceptable. This is where a kid is picked on for no other reason than he/she is the runt in the class, has a disability, has a genius level IQ, shy or awkward, or just looks weird. Usually this goes beyond the normal picking and ends up with the kid being physically attacked on a regular, perhaps daily basis. The kid cannot even walk down the hall without hearing insults and derogatory comments hurled at him. Then there is the whisper campaign behind his back.
And as we've seen numerous videos over the years, bullying becase of race (almost always going just one way) has become very common, and usually involves violence while others film it.
Nowadays it also includes endless stalking and abuse via social media. Girls are very suspectable to this because of how much influence social media has over them and their self esteem.
This is where it crosses the line from bullying into abuse and outright torture. It is understandable why some kids have snapped and bought a gun to school and shot their tormenters.
"If you will not fight for right when you can easily win without blood shed; if you will not fight when your victory is sure and not too costly; you may come to the moment when you will have to fight with all the odds against you and only a precarious chance of survival. There may even be a worse case. You may have to fight when there is no hope of victory, because it is better to perish than to live as slaves." - Sir Winston Churchill