Callum McLaughlin's second-half try helped RBAI to a 10-6 victory over Coleraine Inst in Tuesday's Schools' Cup semi-final at Kingspan Stadium.
Callum Smith fired over a long-range penalty to put Coleraine ahead before TJ Morris also slotted between the posts to leave it 3-3 at the break.
McLaughlin sprinted clear for a converted try and RBAI held on to win after a second Smith penalty.
Methodist College take on Wallace HS in the other semi-final on Thursday.
Methody are going for a fourth straight Schools' Cup success and will start as favourites against Wallace at Kingspan Stadium.
The match will be a repeat of last year's semi-final, which Methody won 17-12 at Queen's.