The National Labor College, an education venture for working adults supported by the AFL-CIO, will close next year because of financial difficulties school officials attribute in part to the construction of a conference center several years ago on the Silver Spring campus. The college, with 599 online students this fall, announced Wednesday that its board of trustees voted this week to accept a closure plan. Word of the impending shutdown had been circulating since at least mid-November. “We’re all very, very sad,” said Paula E. Peinovich, the college’s president. The college will offer courses through the spring semester and have...