A stunning third-quarter performance, that saw them turn a two-point interval deficit into a five-point gain, proved decisive for Castlecomer in Bellefield.
A succession of frees from Billy Dunne, son of Wexford manager Liam, helped St Peter's to a 0-7 to 0-5 half-time lead, with Shane Kinsella's placed-ball expertise keeping Castlecomer in touch.
The winners exploded into the second half, with a Kinsella free followed by a goal from Ollie Feehan in a goalmouth scramble. Eoin Brennan, Feehan and Podge Delaney tagged on points to put them 1-9 to 0-7 ahead on 42 minutes.
Dunne reduced the deficit to two points as the game moved into additional time, but a Kinsella free sealed victory for Castlecomer.
Scorers – Castlecomer CS: S Kinsella 0-7f, O Feehan 1-1, E Brennan, P Delaney 0-2 each; S Rice 0-1. St Peter's College: B Dunne 0-11f, M Joyce, D Shanley 0-1 each.
Castlecomer CS – J Shore; P Brennan, N Feehan, A Dowling; C Delaney, C Wallace, B Coss; D Shortle, E Brennan; S Kinsella, P Delaney, S Rice; D Shore, D Brennan, O Feehan.
St Peter's College – O O'Leary; J Rossiter, S Donohoe, J Fenlon; M Joyce, P Foley, A Maddock; C Devereux, J O'Connor; D Shanley, P Wafer, D Kelly; B Dunne, B Eviston, H O'Connor. Sub: J Firman for Kelly.
Ref – P O'Dwyer (Carlow).