Reigning All-Ireland champions, St Mary's were made work right up to the full-time whistle to advance to the semi-finals.
Mary's, managed by Offaly boss Emmet McDonnell, proved their mettle with a workmanlike display in Mullingar. A Cian McMonagle goal just before the half-time whistle was crucial and it left the Offaly school 1-3 to 0-4 ahead at half-time.
A dogged contest was in the balance with 15 minutes to go as St Mary's sprung Ruairi McNamee from the bench. McNamee and McMonagle were both exceptional.
Athlone held sway until the goal, but then couldn't break down a spirited Mary's backline. Eoin McCormack kept them in touch but their attacks all too often broke down.
Scorers – St Mary's: C McMonagle 1-4 (3f), S Doran, P Connolly, O Lawton, R McNamee 0-1 each. Athlone CC: E McCormack 0-4f, R Daly 0-2 (1f), E Fallon and, K Bannon 0-1 each.
St Mary's – C Hurley; A Mahon, M Kearney, D Leech; S Doran, S Moriarty, D Kennedy; R Caffrey, L Flynn; M Cleer, C McMonagle, P Connolly; I McKeon, O Lawton, D Pyke. Subs: G Doran for D Kennedy (29), J Muireann for O Lawton (30), R McNamee for P Connolly (45).
Athlone CC – E Slevin; K Beirne, M McMahon, M Flannery; C Mulry, E Fallon, K Dolan; S Farrell, R Connellan; K Bannon, E McCormack, R Daly; M Darling, J Quinn, K Dolan. Subs: E Kearney for K Beirne (26), B Shaughnessy for K Dolan (54), J Anders for E Fallon (58).
Ref – C Dourneen (Cavan)