Randi Weingarten, president of the nation’s second-largest teachers’ union, plans to call on Thursday for a full reopening of the nation’s schools for the next academic year, saying: “There is no doubt: Schools must be open. In person. Five days a week.”... Teachers’ unions have been one key barrier to a broader opening this school year, accused of slowing reopening timelines as they sought strict virus mitigation measures, even after teachers began to be vaccinated in large numbers. “It’s not risk-free,” Ms. Weingarten, president of the American Federation of Teachers, which has 1.7 million members, plans to say, according to the remarks. She is expected to argue that the health risks can be managed through a range of practices — some of them relatively simple, such as masking and handwashing, and some of them more difficult to achieve at scale, such as decreasing class sizes to maintain distance and procuring additional spaces to meet outside cramped school buildings.