Tensions are rising in three North Shore communities where teachers remain on strike in pursuit of better wages and changes to working conditions.Unions in Beverly and Gloucester voted to authorize a strike on Nov. 7 and schools were closed starting Nov. 8. A third district, Marblehead, voted to go on strike Tuesday.In Gloucester, union members held a rolling rally through the city on Friday. The demonstration followed some heated moments on Thursday night outside the school where negotiations are taking place, an area that’s generally been neutral territory.Union members began chanting as the bargaining teams were leaving. As Gloucester Mayor Greg Verga drove off, video captured him displaying the middle finger.”At this point, my frustrations got the best of me, and I responded to the nastiness being hurled at me trying to make my way home to my frightened wife,” he said.Verga said a former public safety official was shouting …