Kelso jumped out to a 21-0 lead in the first half, then almost shut out Prairie in the second half to keep its playoff hopes on solid ground.
Kelso can clinch a state preliminary round berth with a win over Camas next week, or a Mountain View win over Columbia River.
A River win next week would not eliminate Kelso, but would create a three-way tie for second place with just three teams earning playoff spots.
Colin Seitz threw two touchdowns passes to Tyler Vea for Prairie.
Next up: Kelso (6-2, 4-1) at Union; Prairie (2-6, 1-5) vs. Battle Ground