Look over the previous few years of the high school yearbook. The staff should make a list of all the things they would like to keep and all the things they should throw out. This will help you narrow down where you are headed with your theme.
Consider your school color and mascot. Ask yourself if you could fit these colors into a particular theme. Decide if you want to stick with the school colors in the yearbook theme or if you want to head off in another direction colorwise. Likewise, decide if you plan to incorporate the mascot into the theme.
Review the last year at your school. Think if anything special happened during the past year. Make a list of accomplishments and trying times. Brainstorm a list of adjectives that describe the events of the past year. These adjectives may spur you in the direction of the right yearbook theme.
Keep in mind the culture of your school. Be sure the theme that you choose fits the overall population of students at your school, not just a chosen few.
Pour over the literature you should have received from your yearbook representative. This literature shows popular themes and suggestions for choosing your own theme. Your yearbook rep should be your first phone call when things start to go wrong!
Narrow your possible themes down to a list of 8 and then take a vote amongst the staff. Take the top 4 and run the vote again. Keep going until you have chosen the number 1 theme for your yearbook.