Wallace High beat holders Methodist College 12-10 in an exciting Schools' Cup semi-final on Thursday.
The Lisburn school, who have never won the competition, will play RBAI in the final on St Patrick's Day.
Wallace took the lead through a fine try by full-back Philip Hylands with Andrew Cordosi landing the conversion.
Tries by skipper Saul Herdman and Conor Kelly put Methody 10-7 up but Wallace winger Ben Finlay scored the match-winning touchdown.
Full-back Robert Lyttle had a chance to snatch victory for Methody with the last kick of the game but his difficult penalty from the left touchline went wide.
Methody had been going for a fourth consecutive Schools' Cup success.
Wallace have been in four previous finals - but lost all of them.