Oops - didn't see the other great answers when I started typing this...
Start with an outline of the points that you want to make, e.g.,
- School should be a place we enjoy coming to
- Participating in more national competitions will provide more opportunities for our students
Next outline the reasons you are qualified to be school president.
Last, turn it into a speech:
- Introduction, with a message you're going to come back to at the end of your speech.
- Elaborate on the points you want to make
- Tell us what you're going to do to make them happen
- Tell us the reasons you're qualified
- Conclude the speech by getting back to the initial message.
Keep your speech on track & it will be easy for people to follow.
Good luck :-)