With many of the Saints lineup yet to turn 18, some were facing opponents up to three years their senior, but it didn't show as they defied a late surge to hold onto a deserved victory. Sam Olver's boot proved to be the difference after his try and two others from Howard Packman and Chris Lord made it three-all in that department.
With both sides flying into the tackles in a physical opening 10 minutes, possession was fairly even. Territory, however, was not. Leinster were keeping the Saints pinned inside their own half and when the chance came, Jack Fitzpatrick broke the line and scored for the hosts.
The response came quickly from the Saints and while a good move broke down on the left, Sam Olver's piercing break and backhand offload to Tom Collins had Leinster cut wide open. The centre's pass however was then slapped down and with Danny Hobbs-Awoyemi unable to collect an ambitious chip through, play came back for Olver to kick a simple three-pointer.
The points were almost immediately cancelled out, but Thomas Daly's penalty was too close to call over the top of the upright and the Saints turned the 22 dropout into a promising counter. Fred Baker showed good pace when hitting the gap and with another pass knocked down by Leinster, Olver captured the lead with a well struck penalty.
It was all Saints now and Ruriadh Smith picked up on a loose ball to streak clear, before playing in Howard Packman with a well timed pass. The extras were added by Olver and the Saints had opened up a handy lead after a tricky start.
Momentum soon switched back with the home team but they were proving their own worst enemies. Centre Daly missed his third kick of the day and when another penalty was awarded for not rolling away, a new kicker in Steve Crosbie didn't bring about a change in fortunes as he too fired wide.
Luck wasn't helping the Saints relieve the pressure, Ashley Elvers and Fred Baker both leaving the fray injured in quick succession, but the half-time whistle came with no further points for either side.
The patched up Saints regrouped and came out firing after the break and after a few minutes of pressing the Leinster backline, Jordan Husband found room on a winding arc. His good work allowed the ball to be shipped quickly to Olver who made no mistake with an accomplished finish.
Leinster needed to get on the board again quickly in order to cling onto the Saints coat tails and they duly did, Crosbie atoning for his error from the tee to latch onto a basketball style pass from Joshua van der Flier.
Just as they had early in the first-half however, the Saints seemed to spring back at the hosts and Olver increased his haul to 16 points with a third penalty of the afternoon. There was barely time to breath before Leinster were back in the points and it was a role reversal of their earlier score, Crosbie setting up van deer Flier to take the try count to three-one in his side's favour and the successful conversion made it a two-point ball game.
The Saints were certainly being made to dip into their reserves as two more players were forced from the field with worrying looking knocks but it was to their credit that the performance didn't drop despite so many changes.
With the clock ticking down, the Saints made the most of another foray into Leinster territory. While the decision to run a kickable penalty may have backfired on another day, Collins' burst put his side within touching distance and with men over, the ball was spread efficiently to put Gabrielle Carroll away in the corner.
With Olver narrowly wide with the extras, a converted score for Leinster was still enough for a tie and the onslaught began. The Saints defence was heroic to keep Leinster out and turnover, but when the kick that would have ended the game was charged down, they had to do it all over again. Thankfully, second time round it was the same result and the joy was clear to see as the Saints youngsters celebrated a superb win.