Why ModelState.IsValid value getting false

[HttpPost]
       public ActionResult StudentInfo(StudentForValidation studentval, IEnumerable<HttpPostedFileBase> Photo)
       {
           //Redirect to partial view
           // RedirectToAction("~/StudentInfo/Index");

           //write this code on partial view submit
           try
           {
               if (Photo != null)
               {
                   foreach (var img in Photo)
                   {
                       var fileName = Path.GetFileName(img.FileName);
                       var Filepath = Path.Combine(Server.MapPath("/Images/Student"), fileName);
                       img.SaveAs(Filepath);

                       // string photoUrl = @"../../Images/" + fileName;

                   }
               }

               if (ModelState.IsValid)
               {

                   student s = new student();
                   if (studentval.Birthdate >= DateTime.Now)
                   {
                       ModelState.AddModelError("","Birthdate Is Wrong");
                       return View();
                   }
                   if (studentval.Middle_Name != studentval.Parent_First_Name)
                   {
                       ModelState.AddModelError("","Your First Name Mismatch with Student's Middle Name");
                   }

                   s.First_Name = studentval.First_Name;
                   s.Middle_Name = studentval.Middle_Name;
                   s.Last_Name = studentval.Last_Name;
                   s.Address_Line1 = studentval.Address_Line1;
                   s.Address_Line2 = studentval.Address_Line2;
                   s.Address_Line3 = studentval.Address_Line3;
                   s.Birthdate = studentval.Birthdate;
                   s.City = studentval.City;
                   s.Country = studentval.Country;
                   s.State=studentval.State;
                   s.Distance_From_Home = studentval.Distance_From_Home;
                   s.Division = studentval.Division;
                   s.Gender = studentval.Division;
                   s.Hobbies = studentval.Hobbies;
                   s.Languages_Known = studentval.Languages_Known;


                   parent p = new parent();
                   login l = new login();

                   smEntity.students.Add(s);
                   smEntity.SaveChanges();

                   smEntity.parents.Add(p);
                   smEntity.SaveChanges();

                   //create format for username
                   l.User_Name = s.First_Name + "@1234";
                   l.Password = s.First_Name + "@" + s.Birthdate;
                   smEntity.logins.Add(l);
                   smEntity.SaveChanges();
               }
           }
           catch (Exception e)
           { }


           var error = smEntity.GetValidationErrors();
           int count = error.Count();

           RedirectToAction("Index","Index");
           return View();

       }