Schools' Cup holders RBAI are through to the St Patrick's Day decider thanks to a 27-19 win over Ballymena Academy at Kingspan Stadium on Monday.
David Forsythe's try put Ballymena ahead before their Belfast opponents hit back with tries from David Whitten, Conor Field and Michael Lowry.
RBAI led 19-7 at the break and Mark Keane added their fourth try before Ballymena staged a comeback.
Mark Thompson and Christopher McLeister went over but RBAI held on to win.
RBAI will play the winners of Tuesday's other semi-final between Campbell College and Dalriada.