A Billerica man is charged with robbing a Lowell bank and fleeing on foot towards an elementary school Monday morning, police said. Robert Volis, 39, is accused of making off with an undetermined amount of cash from the TD Bank on Westford Street. Police arriving at the bank obtained a description of the robber and learned that he might be armed and headed toward the Charles W. Morey Elementary School, said Lieutenant Kelly Richardson of the Lowell police. Volis was apprehended in the school’s parking lot, police said. He pleaded not guilty at his arraignment in Lowell District Court Tuesday on an armed robbery charge and was held on $10,000 cash bail.