An Iowa liberal arts college is challenging a vote by student workers to expand their union. The appeal puts Grinnell College and its student dining workers’ union in the thick of a national labor law debate. Grinnell College is asking national regulators to void an expansion vote by the campus’ Union of Grinnell Student Dining Workers. Administrators had indicated they’d appeal to the National Labor Relations Board after union members voted overwhelmingly to add all student workers to their rolls.