The Holliston Parent Teacher Student Association will hold a special meeting at 7 p.m. Tuesday to discuss how to secure enough participation to ensure the volunteer organization’s future. Membership is robust at the elementary school level, but has been declining at all levels for several years, according to a posting on the school system’s website, www.holliston.k12.ma.us. The Holliston PTSA, the umbrella organization for the chapters at each school in the system, had to cancel its annual fund-raising auction this year due to the lack of volunteers, according to the note. Without the proceeds from its largest benefit event, the group will not be able to fully fund several programs in the schools, including teacher grants, parent education forums, and scholarships for graduating seniors. The public gathering to discuss the volunteerism and leadership issues will be held at the Holliston High School library.