In an entertaining game at Springfield, Grouville ran into a two-goal lead before their midfielder Sam Dustow lashed a penalty kick over a crossbar.
Nine minutes after the resumption St Ouen were back in the game at 2-2 only for Grouville to then net a couple of quick goals on the hour to restore their two-goal cushion.
Grouville added a late fifth to complete the scoring.
Chris Sharp (two), Aaron Bowen, Daniel Hegarty and Henry Adelmann netted for the winners with the later also scoring an own goal before Rhys Le Quélénec made it two-all.
The U16 Division II title will be decided on Sunday 30 April.
In the final divisional match, St Ouen host leaders Sporting Academics, who they trail by two points.