Depends on the market. I would make separate ads appealing to Students, Mentors, and Businesses.
Students: Something catchy and exciting, full of color. Probably envoke a rough edged/grunge style, that seems to be the popular style at the moment.
Example:
Mentors: Collegiate poster, with the benefits of helping such a program. Informative but eye-catching at the same time.
Sponsors: Very professional, annual-report style. Clean, simple, to the point. Probably a lot of white space.
Example:
