BOSTON—A mass of arctic air swept into the Northeast on Tuesday, bringing bone-chilling sub-zero temperatures and closing schools across the region for the second time in less than a week. Schools in Massachusetts three largest cities—Boston, Worcester, and Springfield—canceled classes, saying they did not want children standing outside for extended periods of time waiting for buses. “There has been an increase of covid with transportation personnel, which would result in buses running up to 30 minutes late,” according to a tweet from the Worcester public schools. “The safety of our students and staff are always the focus of our decisions.” Low temperatures can result in frostbite to exposed skin in as little as 30 minutes, according to the National Weather Service. Syracuse, New York, Manchester, New Hampshire, and Burlington, Vermont, were among communities that also closed public schools. The closures came just a few days after many school closed because …