A discussion about diversity in Lexington public schools will highlight Lexington’s 20th annual CommUNITY Commemoration of Martin Luther King Jr.’s life and legacy Sunday . The commemoration will begin at 1:30 p.m. with a walk from the Lexington Battle Green to Cary Memorial Hall, at 1605 Massachusetts Ave., where the indoor program will begin at 2 p.m. Robert J. Harris, assistant superintendent of human resources for Lexington public schools, will open a panel discussion about diversity in the school system. Several local educators will join in the panel discussion, along with Malisk Alfred, a 2012 graduate of Lexington High School, who was in the Metco program and is now enrolled at Dean College. In honor of the town’s 300th anniversary, the commemoration will also discuss Lexington’s historical demographic changes. Admission is free, and the grass-roots Lexington CommUNITY Group is encouraging people to participate in the walk from the Battle Green.