A recently published study argues that for at least 25% of overweight kids bullying keeps them off their school playgrounds:

Overweight kids can be bullied or taunted right off the playgrounds or ball fields, potentially making their struggles with excess pounds even worse, researchers here said.
One quarter of overweight children ages eight to 18 reported significant problems with bullying, and such problems correlated strongly with self-reported depression, loneliness, anxiety, and curtailed physical activity…

First, let’s all agree that bullying is bad behaviour and should not be condoned. Second, I’ll go so far as to say that bullies, any one who threatens to or actually initiates physical violence against another student, should be removed from the school and face appropriate criminal charges.
Now, what really keeps kids off playgrounds and also forces them into an environment where they have to deal with bullies on the playground? Well, our school systems. That right.
The school system herds large numbers of students together into large buildings for a significant part of there day. During that day these young, energetic people spend most of their time crammed into small rooms, sitting in rows of chairs, sometimes behind desks. Five-six hours a day like this and we wonder why many don’t get enough exercise. Then during the day elementary* school kids may be released with 100’s of their schoolmates into a large caged yard, one that is not unlike a prison yard. Hmmmm, energy, 100s of kids, some of whom are jerks and bullies, in a large caged area. Just what kind of behavior do you expect?
Yes, damn the bullies. But also damn the institutions that force all these people into this overcrowded environment. Unless, that is, you really want to train our children for a life in prison or sweatshops.

*Note that by junior high and high school recess is a thing of the past and the bullies do most of their preying in the hallways, at lunch time or after school.