Fat kids don't feel good about themselves...

...while kids who do well on exams do feel good about themselves.

According to this study from the University of Alberta:

Kids who scored well on provincial exams tended to have high self-esteem, the researchers said, although children with low self-worth still managed to perform well. Children dealing with obesity, meanwhile, weren't feeling good about themselves.
"Body weight affected self-esteem negatively, and ... school performance affected self-esteem positively," the study says.

For another article on the same study, go here.