NEWTON — A Newton elementary school has canceled classes for the day after two employees were overcome by carbon monoxide.The fire department says the employees were overcome by fumes in the Franklin Elementary School's boiler room at about 7:30 a.m. Friday and taken to a Boston hospital.Firefighters searched the whole building and found no one else inside.Testing, however, showed unsafe levels of carbon monoxide in the building.The cause remains under investigation.No names were released.