RBAI ran in seven tries to secure a 41-8 victory over Ballymena Academy in the Schools' Cup semi-final at Kingspan Stadium on Monday afternoon.
Tries from Zak Davidson and Niall Armstrong helped the holders to a 12-0 advantage at the break.
The Belfast team surged clear in the second with Armstrong adding two more tries while Yasser Omar, Michael Lowry and Callum Reid also touched down.
Ballymena did manage to register a try through Azur Allison.
RBAI will play the winners of the second semi-final between Methodist College Belfast and Belfast Royal Academy at the same venue on Tuesday.
Belfast Royal Academy were awarded a walkover in the quarter-finals after opponent Friends' were unable to field a team, with 17 of the their squad on a skiing trip in Austria.