It was a balmy August morning in Santa Monica, Calif., and the city bus barreling down Olympic Boulevard toward the Pacific Ocean was packed. High school students crowded almost every seat, sandwiched in between a handful of actors and a movie director on a ride to nowhere.

The bus had been transformed into a rolling movie set, borrowed from the city by a group of high school filmmakers in town from London. A thick padded blanket was taped to the ceiling to separate the crew, in back, from the actors up front. Toward the rear, Tony Fernandes, an 18-year-old from North London and the movie’s director, watched a scene on a monitor of a man trying to persuade a German tourist to take his photograph.

Also onboard was Jan de Bont, an action director who knows a thing or two about how to make a movie on a bus. He directed “Speed,” the 1994 Keanu Reeves blockbuster, and was there to advise the teenage crew. The bus stopped at a light, lurched around a corner, then stopped again. “This bus needs to keep moving,” Mr. de Bont warned the young director. “If not, it looks weird.”

Mr. Fernandes reshot the scene, this time in motion. “That’s better,” Mr. De Bont said. “It has to look natural.” The bus pulled up to the curb in front of a Burger King, and a local woman banged on the door to be let in, sending a quiver of giggles through the aisle. It was nearly noon, time to return to the bus yard before heading to a new location and another shoot.