The top-seeded Rebels (20-5) will play Whitney or Linfield Christian on Friday at a site to be determined.
They easily advanced.
"And that's what made it fun," Flintridge Prep Coach Garrett Ohara said.
They had fun from the start.
Flintridge Prep had unanswered scoring runs of nine, 10 and 11 points in the opening 10 minutes as it built a 25-4 lead, thanks to its stifling defense and potent offense. St. Paul (6-21) made just one of its first 15 shots, and the Rebels hit nine of their first 10 shots.
"We knew we were going to go hard and give every bit of our energy," said Harvard-bound Rebel senior Kenyatta Smith, who had a game-high 17 points and 15 rebounds in three quarters.
"We wanted to prove we were the No. 1 seed."
Added Rebels freshman Robert Cartwright, who had 13 points, four rebounds and three assists: "We couldn't miss a shot."
By halftime, the Rebels had a 31-point lead and the starters played the third quarter before giving way to the bench.
Jedrick Eugenio had 15 points and Jared Norsworthy and Jordan Whaley each had 10 points for the Rebels. Flintridge Prep, which shared the Prep League title with Pasadena Poly, had 51 points in the first half, tying the season-high mark it set in a 103-54 victory against Covina on Jan. 22.
The 36-point margin of victory was nothing new for the Rebels. It was their ninth win of more than 30 points this season.
Said Cartwright: "We're going into the CIF playoffs to make a run at the championship."