To avoid self-selection, have all students at the school try CS. This can help provide all students a background in CS so that they can see if it is something they like and might want to pursue. Students who don’t fit the stereotype of a typical computer scientist often opt out of trying CS even though they might really like it! Try to focus your efforts at your school toward programs that serve all students.
Example: "CS is helpful to a range of jobs!"
See this and related tips as a Tip Sheet: http://csteachingtips.org/tips-for-recruitment-in-HS