SCOTTS VALLEY, Calif.—Skateboarders often thumb their noses at the rules. At the local skate park here, some teens defy park guidelines by skateboarding without a helmet or safety pads.

But there was one rule many liked to see enforced: that Scotts Valley Skate Park be used for "skateboards, in-line skates and roller skates only."

So last December, when hordes of younger boys atop kick scooters—those two-wheeled aluminum planks driven by handlebars—invaded the venue, they invoked that rule to a tee. Police were called in and at least 50 scofflaw scooterers were kicked out in one day.

Across the nation, a skateboarder-scooterer divide is showing up—and in the process is turning skateboarders' antiauthoritarian conventions upside down. That's putting many town halls, law enforcement officials and park commissions in a tough spot, as they are now being pushed to decide: Can scooters coexist with skateboarders?

It's a debate that officials in several cities are wrestling with.