This is when a couple has started dating in high school. Some teenagers treat their relationships like the end-all and be-all of the world, and sometimes the author agrees, implying or directly showing that they marry and live Happily Ever After. Obviously, this rarely happens in Real Life nowadays, but "rarely" means it's still pretty much Truth in Television (just not dominant). Fiction writers might do this to avoid addressing the sad fact that the lovely couple making out at the end of a teen romance story would probably break up after high school in the real world, especially if one or both parties go to college.