Clash Day is part of EA's Spirit Week as the school prepares for Saturday's Episcopal-Haverford School Day.
Buckley, assisted by Michael Hinckley, buried a shot into the top of the net from 15 yards. Grims scored off an assist from Raphael Furey.
It was the second straight season that the Churchmen, the 11th seeds, upset a first-round opponent. Mercersburg was No. 6.
"We didn't travel six hours round-trip to come away with a loss," Knox said.
George School 3, Penn Charter 1 - John Arth and Ethan Lamb scored in the first half, and Luke Haug added two assists for the fifth seeds against the 12th-seeded Quakers.
PIAA Class AAA First Round
Chad Fluharty scored in the first half and Evan Hermans added a late goal in the second half to seal Downingtown West's 2-0 win over Abington Heights in Scranton.
Luke Elicker set up Fluharty, a sophomore, for the goal from 12 yards. Hermans scored from close range off an assist from Luke Maruca. Senior Josh Brain earned the shutout with seven saves.
Great Valley 2, Central York 1 - Michael Sprinkel scored with 20 minutes left to lift the Patriots at West Chester East. Great Valley fell behind, 1-0, early before Jake Miller evened it on a first-half penalty kick.
Conestoga 1, Parkland 0 - Thijs Everts broke a scoreless drought with two minutes left, and the Pioneers held on to win at J. Birney Crum Stadium in Allentown.
PIAA Class AA First Round
Holy Ghost Prep used four goal scorers to ease past Lansdale Catholic, 4-0, at Council Rock North. Tim French, Mike Kirby, Pablo Platero, and Adam Snyder scored for the Firebirds, who scored twice in each half.
PIAA Class A First Round
Alan Smith scored three times and Michael Lewis added a goal to lead New Hope-Solebury over Conwell-Egan, 4-0, at Northeast.