An effective campaign speech has three important components that must be included.
Tell the audience who you are and what office you want to hold. Most people try to do this in a humorous way; for example, "Hi, I'm John Smith. I usually run for the track team, but right now I'm running for Student Council President." If you just can't think of something funny, don't force it. You want to sound sincere, not fake. You should also tell how long you've been at the school, what grade you're in, and any other offices you've held. This will establish your credibility. If you've only been at the school for a few weeks, the other students might think you don't know enough about the school, but if you can cite examples of your accomplishments at your previous school, you can convince them that you can help at this school, too.
This is where you explain why you are running for office. Outline the problems you've identified and explain how you would attempt to solve them. You should have more than one issue to address, but don't go overboard. If you list too many problems, you'll sound negative and whiny. Everybody knows their school has problems, but school loyalty is important. You should emphasize that you want to make things better--not completely change the school.
Finish your speech by thanking your audience for their time and attention, and tell them how much you are looking forward to serving as a member of the student government. Invite them to talk with you about their concerns, and remind them that the only way to make positive changes in the school is by being involved in the decision-making process.