Tell them if you get elected, you can convince the school to invite (insert well-known Football star here) to your school and talk about exercise and play tag-touch Football. Unless your school doesn't have a Football field.
Become popular, I know how you feel. Most of the kids in my grade are total idiots. And guess who are the school officers? The popular idiots. Luckily the most popular one's pretty smart, but when he doesn't run, it's no wonder nothing gets done at our school. And to anyone about to say, if your so smart why don't you run? Well I have, and it's unanimous, friends vote friends, and I probably have the least friends out of anyone in my school. And I like that. Although it makes me laugh watching kids who are half failing running your school. So make people like you, it has little to do with real qualities untill you are elected, sad fact of life. Good luck.
Since you are thirteen and you will be making a speech in front of 3rd-8th graders, I would keep it simple. Don't make it too drawn out and boring. Make sure not to talk too fast and speak loud. I say a thirty second speech will do the trick. Lie, if you have too. I doubt you will have too much power at your age in your school's doings. You could be like the guy who won Senior Class President at my school by going up there and saying