As the clock ticked over into the 5th minute of added time in the second half, it was RBAI captain and Player of the Match Jacob Boyd who stormed over for the winning try.
It had been gridlocked at 17-17, and looked almost certain to be heading for a draw which would have seen the trophy shared for the first time since 1996.
Campbell had been the better side in the early exchanges on a wet and windy St Patrick's Day, forcing Inst to get through plenty of defensive work in their own half.
With 19 minutes played, Campbell took the lead through a well-executed lineout maul which saw hooker Jed Dornan touch down for the first points of the game, with Matt Rea converting to give them a 7-0 lead.
Inst responded 6 minutes later after some creative work saw wing Flynn Morrow join the scrum, which freed up powerful number 8 Bryn Ward to make some hard yards with another rampaging run in midfield.
This set the stage for a series of quick drives at the line from close range, and it was captain Jacob Boyd who muscled over for his first try.
A penalty from Campbell's Rea was the final act of the first half, and gave his school a 10-5 lead.
Early in the second half, RBAI took the game to Campbell and soon scored to claim the lead for the first time in the final.
Jacob Boyd and Byrn Ward were once again used to make driving runs at the Campbell line, before lock Jacob Edwards powered over from close range.
He touched down close to the posts, and Josh Eagleson made no mistake with his conversion to make it 10-12 to Inst.
The momentum looked to have fully swung in favour of RBAI.
They then worked the ball from a scrum on the right just inside the 22, moving it through the hands to Josh Eagleson, who launched a superb long pass out wide to winger Alex Place to dive into the corner to score.
Place's effort made it 10-17 but Campbell soon fought back and got themselves level.
The impressive Lukas Kenny gathered a clearance kick with 59 minutes on the clock and skipped his way past 2 defenders before releasing Jake Daly.
The left winger then found right winger Henry Ralston with a pass, and he stepped inside his defender to reach for the line and score. Matt Rea confidently stepped up and split the posts to make it 17-17, and nothing could separate the sides until the very end.
Four minutes into added time, Bryn Ward made another powerful surge, smashing through several tackles on the Campbell 22.
Jacob Boyd then hit the line at full speed, gathering Callum Soper's quick pass and rampaging toward the whitewash to score one of the most memorable tries in an incredible Danske Bank Schools' Cup Final.
<blockquote class="twitter-tweet"><p lang="en" dir="ltr">What a finish! 😮<br><br>RBAI win the Schools' Cup Final in injury time 🏆🏉<a href="https://twitter.com/hashtag/BBCRugby?src=hash&ref_src=twsrc%5Etfw">#BBCRugby</a></p>— BBC SPORT NI (@BBCSPORTNI) <a href="https://twitter.com/BBCSPORTNI/status/1636769117293232137?ref_src=twsrc%5Etfw">March 17, 2023</a></blockquote> <script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>
Teams
Campbell College: Lukas Kenny; Henry Ralston, Matthew Booth, James McConnell, Jake Daly; Matt Rea, Sean Robinson; Chris Massey, Jed Dornan, Flynn Longstaff (captain); Alexander Brennan, Daniel Harkin; Cal McKinney, Cameron Faith, Joe Jeffery.
Replacements: Will Alexander, Robbie Holmes, Daniel Solomon, Charlie McClurg, Scott Gray, Harry Dyer, Sam Boomer, Jay Davis.
RBAI: Joshua Gibson; Flynn Morrow, James Hilman, Fraser Cunningham, Alex Place; Josh Eagleson, Callum Soper; Blake McClean, James Clark, Jacob Boyd (captain); Jacob Edwards, Daniel Moore; Callum Simms, Jack Parkinson, Bryn Ward.
Replacements: Euan Paterson, Zak Molyneaux, Shéa Scullion, Milo Carter, Loughlin Sweeney, Sam Patterson, Matthew Sweetlove, Adam Hawthorne.