If you were a fat kid, you can relate quite well to what I'm about to talk about. I'm going to discuss the agony of gym classes in school. Even if a fat kid happens to be not so terrible at some activities or sports, there's so much indignity involved in the classes that they'll learn to hate them sooner rather than later.
Being fat in gym class means being picked last when teams are chosen (unless one of your friends gets to make the choices and chooses you out of loyalty). There's nothing like always being treated as the least desirable person to help tear down any remaining self-esteem that is left after having to wear shorts in front of everyone and show off your fat legs. What is worse, of course, is community showering where people get to see your entire wobbly body in a manner which violates your privacy and puts you on display for others to laugh at.
Of course, being fat does usually mean you're slow and probably not the greatest at sports, and that's all the more reason why gym classes shouldn't be about competition as much as health. I never understood why my gym classes were centered around things like softball, kickball, dodge ball, etc. Those sports actually ensured that we spent less time being active because they were turn based. You spent a lot of time waiting for your turn, acted on your turn, and ran a bit then started the whole process all over again.
What was worse than the competitive sports though was the absurdity of doing gymnastics activities. I'm not sure how walking along a balance beam, jumping over a pummel horse, etc. was meant to make my young body fit, but it sure did make me feel inadequate and hate every gym class. Instead of encouraging fitness, they tested for it and it was a test I always failed. If anything, gym discouraged exercise.
I believe that, if schools were really interested in helping kids be active and healthy, they'd offer a variety of options for gym like constantly walking for 30-60 minutes a day. Can you imagine how much better it'd be for overweight kids to spend an hour walking around a track or the gymnasium each day than facing the humiliation of organized sports and fitness-testing activities? At the very least, the option to do this instead of a regular gym class should be offered. I know I would have been happy to spend an hour walking and talking to friends each day instead of taking gym class twice a week. Walking not only would be better for overall fitness, but it'd encourage daily movement and show how you can get helpful exercise without feeling or looking foolish in front of everyone.