MISSION — For some elementary school students, the familiar ice cream truck jingle apparently meant an easy way to buy drugs. Police are stunned that children in the third grade were buying pot from an ice cream truck driver, whom they arrested Thursday afternoon. The driver, police said, was selling $5 and $10 bags of marijuana to students coming out of O’Grady Elementary School at the intersection of Farm-to-Market Road 495 and Holland Avenue. Customers included third-, fourth- and fifth-graders. “I can’t visualize any of these kids that age smoking marijuana,” Mission police Chief Leo Longoria said. “It’s a scary...