What motivates high school students to join professional organizations?
getting higher credits
participating in competitive events
getting scholarships to further their education
paying lower membership dues

Getting scholarships to further their education, motivates high school students to join professional organizations  

Explanation: The reason is because a person is able to work legally when he/she turns 18 and there are many students in the United States, who want to get further education but cannot due to financial constraints. Thus, many students are motivated to join a professional organization to save money and get  scholarships for their higher studies.