Methodist College Belfast won 19-3 over Wallace High to set up a Danske Bank Schools' Cup final meeting with Campbell College Belfast on March 18.
The Belfast school, the favourites for the title, triumphed thanks to a brace from openside flanker Adam Reid and a further score from full-back Ethan McIlroy, who kicked another four points from the tee.
Wallace's sole score came from a second-half penalty from scrum-half Nathan Doak, son of former Ulster player Neil.
But on the stroke of half-time, the competition's most successful side forged ahead when Ulster U19 winger Chris Larmour surged up to just shy of the line, with Reid crashing over from close range a few phases later.
Doak reduced the gap early in the second half when he converted a penalty, but that would prove to be their only score as Methody sealed the win.
Reid grabbed his second when he picked and went off the back of a ruck following a strong carry from captain Kevin McNaboe, a score which effectively ended the game as a contest.
And McIlroy added some gloss to the scoreline, the full-back running in after centre John Fletcher made a nice break up the middle and supplied the pass.