A natural gas main in Union City was ruptured by a crew doing work on a water line Thursday morning, forcing the evacuation of students in summer programs at the city high school and and neighborhood residents. Union City police and members of the North Hudson Regional Fire and Rescue and PSE&G responded to the intersection of 26th Street and Summit Avenue at approximately 9:30 a.m. and found gas and water spewing into the air. A Veolia crew was repairing a 6-inch water main when it struck a mismarked gas main, said a spokeswoman for the company that operates the water system for Union City. T…