Kids are cruel to each other. I see it now and I remember it from when I was a "kid". I believe the explanation that kids are cruel because they are children and are not yet socialized.
Children learn from their parents. The parents are setting the role model example for their kids.
Children can be such cruel little creatures because they also don't have any sense of self-worth yet. So they try to gain it by tearing down or controlling everyone around them. They can also become parts of cliques for the security and confidence that being in a group gives. The friction at these groups' boundaries tear outsiders down while protecting the group's members. This gives an impression of superiority and feeds the members' needs for self-worth.
I am referring to elementary, junior high and yes even high school kids can be cruel IMHO.
If they never develop good self esteem about themselves they can grow up to be cruel adults.
I have read some great comments by the members who have already posted.
Great question. It is very hard to watch your child in pain because someone has been cruel to them .