Kids can and are cruel, even though they are 'taught' about 'bullying' in school and supposedly, and I mean supposedly, in the home!
My oldest has had little things through the years. He was nervous, earlier, this year because one of his 'friends' said something about his push-up form. Let's say, I showed him. Used Yoga, Plank Pose, to get him precisely where he needed to be. He needed push ups to start the wrestling club, this past spring. So we started in January.
Guess, their daddy's work them harder than I once, thought...hmmmm...

I know what you mean, about teasing, taunting and being left out.