public static void CreateDataContext()
 {
     m_db     = new SQL_QUANLYHOCSINHDataContext();
     IsLoaded = true;
 }
 public static void CreateDataContext(string connectionString)
 {
     m_db     = new SQL_QUANLYHOCSINHDataContext(connectionString);
     IsLoaded = true;
 }