SACRAMENTO — Police sirens wail as a scruffy teenager, clutching a bag, runs frantically through the streets. Entering a schoolyard, he reaches into the bag. Out comes … a graduation gown, which he dons to receive a diploma. The scene is from a television ad, paid for with tax money and made by consultants close to Hollywood producer Rob Reiner. It aired across California this winter, touting the benefits of preschool. "When kids go," the narrator says, "we all benefit." The release of the ad, and two others, by a state commission Reiner heads coincided with his launch of a...