I was overweight in school, and as a result, the slowest runner in my year (I was as slow as a physically disabled pupil who needed aids to walk - so, it was pretty bad). I tried really hard, attended P.E lessons, and whilst my fitness increased I was always shit at the running.
We had to do the 1.5 mile run infront of the rest of the high school on sports day. As in, they do all sit at the edge of the track, and watch you, taking turns by year. I felt sick to my stomach all day, was worried the night before, knew it was going to be bad (but didn't know it would be as bad as it was). I wasn't very popular either.
What I was surprised to find, was that when I was on my last lap (completely alone with the other pupil mentioned) and clearly struggling, but trying my best, so many people cheered me on by name, and all cheered for me when I finished. They were incredibly kind and encouraging, and it made me think twice about the people I thought would be horrible to me.
One of my friends who was also overweight in school and struggled with fitness, didn't take part on the day - I think she got a sick note. I'm glad I took part, given a person with a very limiting disability also did it, and it would have been so much harder for her, even with her aid, there was no excuse for me to not take part.
That being said, never in my life have I ever needed to complete a run infront of people since (despite quite enjoying a little jog now!) so, if she's attending P.E lessons, doing exercise otherwise, I don't think it's the worse thing in the world to let her sit it out.