The death of a 13-year-old student at his school in Old Washermenpet on Thursday was due to cardiac arrest, doctors said, after a post-mortem was conducted at Government Stanley hospital on Friday. Ganesh Kumar, a class VII student of K.C. Sankaralinga Nadar Higher Secondary School, collapsed during the P.T. period and was rushed to a private hospital and later taken to Stanley hospital where doctors pronounced him brought dead.