The 6-foot-8 Jefferson, a blue-chip recruit expected to announce his college destination soon, played despite tweaking his right ankle in the early going.
Held scoreless in the middle two quarters, he finished with 14 points.
"Our guys were hot," Jefferson said of a torrid shooting effort. "My teammates picked me up when it was needed. We played a total team game."
Friends' Central (22-5) made 10 of 23 attempts from beyond the arc. Guards Billy Cassidy (four three-pointers), Conrad Chambers (three), and Karonn Davis (two) did most of the damage.
"We had a team meeting after losing to them the second time," said Cassidy, a junior sniper who transferred from Monsignor Bonner. "We were dying to get another crack at them. We made it our goal to finish the season by winning states."
Cassidy totaled a game-high 19 points, draining a three-pointer in each quarter. Conrad Chambers, a sophomore guard, was next with 18 points. Junior guard Karonn Davis chipped in 10 points and six rebounds.
Polykoff said that taking the injured Jefferson out of the game was not an option.
"I could see he was limping, but there was no way he was going to let me put him on the bench," Polykoff said.
Daniel Ochefu, a 6-10 center bound for Villanova, and wing guard Jeremy Schulkin paced Westtown (21-10) with 16 points apiece.
Freshman guard Jonathan Lawton, getting a rare start for the Phoenix, guarded Schulkin most of the night; 6-5 senior reserve Shea Norcini spent time defending Ochefu.
"We thought we had a better shot of beating them on a bigger court," Cassidy said. "We like to get out in transition and push the ball. We were able to do that more on this court."
Friends' Central was ahead, 40-30, after three quarters. A 7-0 burst at the start of the fourth, a stretch that included a three-pointer from Cassidy and a lane floater by Jefferson (10 rebounds, two assists), all but clinched the triumph.
Schulkin, who nailed a buzzer-beating three at the end of the first quarter, fouled out with 2 minutes, 31 seconds remaining.
Westtown 11 7 12 17 – 47
Friends' Central 16 11 13 26 – 66
W: Floyd Preito 5, Daniel Ochefu 16, Yilret Yiljep 4, Jeremy Schulkin 16, Leonard Klein 3, Marcus Howell 3.
FC: Karonn Davis 10, Billy Cassidy 19, Amile Jefferson 14, Conrad Chambers 18, Pedro Rangel 2, Franz Hueber 3.
Contact staff writer Rick O'Brien at 610-313-8019, email@example.com, or @ozoneinq on Twitter. Read his blog, "The O'Zone," at www.philly.com/ozone