Cheyenne-Reydon hung close for much of the second half, trailing 57-53 with just over five minutes to play before Glencoe ignited on a 16-3 run to close the game.
Junior Braygan Lippencott had a game-high 26 points to lead Cheyenne-Reydon, which closed the season at 23-7.
“We always battle hard against each other, but I thought it was so classy when we were going through the line shaking hands, all of their guys were hugging our guys and telling us to go win it now,” John Lazenby said. “It's the good side of a rivalry. They're brokenhearted. We're elated. But somebody's got to lose, and that was really nice to see.
Glencoe (29-1) will face the Wright City/Sterling winner at noon Friday in the semifinals at State Fair Arena.