AP Sports School of Andhra Pradesh also maintained their winning streak to beat Nayagram Thana Balika Vidhalaya, Paschim Medinipur (West Bengal) 3-2 in a well-contested match to make the quarterfinals.
Anitha scored a brace for the Andhra team.
V Kumari led St Mary's charge with five goals, while Soubaliya, Rasika, Manabika and Jayarama netted the ball once each.
The Tamil Nadu team are now leading their pool with two wins from as many matches.
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The Afghan team rode on goals from Fahima (12th and 18th) and Arezo (32nd and 36th) as they dominated the proceedings right from the start.
The Delhi team managed to get their act together in the second half and created a couple of chances, but they were no match to the Afghan.
Government Model School, Haryana registered their second consecutive victory by getting the better of Fountainhead School, Gujarat 2-0.