MINNEAPOLIS — Standing outside his son’s elementary school, a father heard a warning crackle over his walkie-talkie: a suspicious car nearby. At another school, volunteer Joe Dirkswager stood watch, nodding at passing vehicles. Miles away in St. Paul, Matthew Kearns was questioning unfamiliar individuals near the school he graduated from decades ago. The three are among the volunteers who have begun patrolling streets around elementary and high schools in the Twin Cities, using walkie-talkies,