As the graduates of Newtown High School filed across an outdoor stage in Sandy Hook, Conn., on Wednesday evening, shaking hands and receiving diplomas, 20 of their classmates were missing.

For the dozens of seniors who had also attended Sandy Hook Elementary School, those classmates have been missing for more than 11 years. They were in first grade on Dec. 14, 2012, when a gunman attacked their school in one of the most horrific mass shootings in U.S. history. Twenty students — ages 6 and 7…