To commit to someone in High School, similar to a marriage, but far less serious because both involved know that they will likely break up once they go to college.
"Katie, will you High School Marry me?"
Person 1: "Dude, Josh and Becca have been going out for a while now. Think they'll actually end up married?"
Person 2: "Nah, they're just High School Married. Josh'll dump her once he starts meeting college chicks."