What kind of yearbook is it? Professionally printed by a company, or hand done and photo-copied by the school? Depending on which it is, there's a big difference on what you can do.
Before you try to think of what to do on the cover, think of a theme to carry out throughout the book and try to embody that theme on the cover. Some popular themes are: the calendar, under water, broadway, secret spy, footsteps in time, and boys versus girls.
