Speeding Tickets: Speeding in a School Zone in Washington State

I received a ticket for speeding in a school zone. I had entered the school zone from a side street where there was no indication that I was entering said school zone, nor were there any children present. When I was pulled over I was told I was going 31mph (normal speed limit is 30mph.) in a school zone (20mph). I honestly had no clue I was in a school zone. There was nothing on the ground, and no signs until the school zone ended, which I couldn't see until I had already been pulled over.

I don't know if this is worth disputing or if I should just deal with my ignorant mistake.

Speeding Tickets: Speeding in a School Zone in Washington State