Holders Methodist College are through to the Schools' Cup decider after beating Wallace High 17-12 at Queen's.
Ryan Branagh's try put Wallace ahead before Michael Lagan went over for Methody just before the break.
It was 7-7 at half-time and the Lisburn lads went back in front through a Philip Hylands try.
Josh Bingham's penalty cut the gap before a converted Steven Weir try won it for Methody, who will play Sullivan Upper or Coleraine Inst in the final.
Sullivan and Coleraine will contest the second semi-final at the same venue on Thursday.