Campbell College beat Dalriada of Ballymoney in Tuesday's Schools' Cup semi-final and will meet Belfast rivals RBAI in the decider on 17 March.
Flanker Jamie Macartney, winger Jack Howard and centre Nicolai Koplewsky scored tries for Campbell in the first half at Kingspan Stadium.
Number eight Jack Barry-Glendinning added a fine try in the second half.
Scrum-half Malcolm Corry landed two conversions while Dalriada's points came from a Stewart Moore penalty.
"Coming here means a lot to both teams," said Campbell coach Brian Robinson.
"It was a physical and well-contested match and Dalriada should be proud of what they have achieved."