Coker, who ran for 87 yards on 19 carries, earlier took in a two-point conversion run after Gabriel Rodriguez's 75-yard punt return for a TD. But Schmidt was just 4-for-14 for 63 yards, all four completions on screen passes, in the inclement conditions.
Schmidt scored Plano's first touchdown, on a 12-yard run.
"We couldn't throw as much as we'd like to," Kunz said. "Our defense kept us in the game. Our two inside linebackers and our two safeties made a lot of plays, and that starts up front."