The Society of News Design has produced an excellent Code of Ethical Standards for visual journalism.
The Public Relations Society of America provides a Member Code of Ethics that can help any publication in developing its own policies.
The New York Times Company has one of the most exhaustive ethics policies, and it applies to everyone employed by the company. This is a great site to browse for ideas that could help you, and language that has been carefully vetted.
The Rock Canyon HS newspaper “The Rock” developed this editorial policy, and it includes commentary from then-adviser Jack Kennedy. The Rock Canyon yearbook, “The Black & Gold,” also developed a one-page editorial policy that can help you get started on your own.