It’s noon on a Thursday, but the day’s lunch break is already over and the cement building in Concord is once again full. Class is in session. A dozen students — most dressed in gray canvas button-downs and baseball caps — sit with rapt attention facing the whiteboard at the front of the room. But the topic of today’s lesson isn’t biology, math or literature. It’s how to fabricate drawings for pipe fitting. The students here are apprentices with United Association Local 342, a union that trains and represents workers in the pipe trades industries. They’ll complete a five-year paid...