"Forty points in two quarters, I've never seen before," said Harrison, whose team fell to 0-2. "We've got a lot of stuff to do.
"We have to get better with our tackling. It was a domino effect. The cards tumbled and the house came down."
Grace Brethren (1-1) controlled the tempo of the contest despite committing 15 penalties for 140 yards.
The Rebels, who owned a 7-6 halftime lead, then saw the Lancers pick on their defensive backs in the fourth quarter. Grace Brethren had touchdown passes of 38 and 56 yards in the fourth quarter.
"Again, it's simply poor tackling," said Harrison, whose team will next face host Campbell Hall at 3 p.m. Friday. "I take the blame for that, but they also will be held accountable. I have to make sure I put the correct 11 out there to do the job."
After Grace Brethren took a 6-0 lead on its first possession, Prep came back to take a 7-6 advantage when senior running back Eric Kazangian scored on a three-yard run on fourth down with 2:23 left in the first quarter.
The Lancers took a 13-7 lead on a five-yard run by Kendall Rettig, who finished with 162 yards rushing in 19 carries. The Rebels tied it at 13 on their next possession when quarterback Kyle McDonald threaded a 49-yard pass to wide receiver Jared Norsworthy.
Rettig then scored two straight touchdowns in the third quarter to give Grace Brethren a 26-13 lead.
The Rebels closed to within 26-20 when McDonald tossed a three-yard pass to Kory Hamane with 10:14 left in the game. However, Grace Brethren closed out the game with three touchdowns in the final 9:00.
McDonald completed eight of 15 passes for 152 yards. The Rebels finished with 60 yards rushing.