public string SaveTeacher(Teacher teacher)
 {
     if (teacherGateway.FindDuplicatEmail(teacher.Email) == null)
     {
         if (teacherGateway.SaveTeacher(teacher) > 0)
         {
             return("Save Successfully.");
         }
         else
         {
             return("Sory! Save Fail.");
         }
     }
     else
     {
         return(teacherGateway.FindDuplicatEmail(teacher.Email));
     }
 }