I went to a high school which was not split by grade. I was called a "geek" or "boffin" (basically the regional equivalent of nerd) because I was often top of the class, or made fun of because I have mild scoliosis. I managed to keep a good group of friends though and I mostly just told the bullies to fuck themselves and got on with my life.
I think there were people who were unable to deal with bullying, and that mostly comes down to a lack of social skills, rather than any extra persecution that they suffered because of their intelligence or appearance.
I would guess it's selection bias.