Contests can create a lot of positive buzz, but the downside is some students won't feel they are good enough to compete. To overcome this, you could make the subject a graded project required of everyone. Each section can then select "finalists" to be presented to the entire school for vote.
The subject matter could be related to a local non-profit agency or even school promotion [tee shirt design, poster for use in high schools, etc. The process is a way to reinforce an important message to the students.
