RelatedObjectDoesNotExist Exception

I tried to define prefix of signing in user, but I got this exception RelatedObjectDoesNotExist. The thing is django cannot find user Student and Parent(Exception Value: User has no student)

I want to render different templates according to user's type

models.py

from django.db import models
from django.contrib.auth.models import User


# Create your models here.


class Teacher(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    prefix = 't'

    def __str__(self):
        return self.user.first_name + ' ' + self.user.last_name


class Student(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    parent = models.ForeignKey('Parent', null=True)
    prefix = 's'

    def __str__(self):
        return self.user.first_name + ' ' + self.user.last_name


class Parent(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    prefix = 'p'

    def __str__(self):
        return self.user.first_name + ' ' + self.user.last_name

views.py

def sign_in(request):
    if request.method == 'POST':
        username = request.POST.get('username')
        password = request.POST.get('password')
        user_login = User.objects.get(username=username)
        user = authenticate(username=username, password=password)
        if user is not None:
            login(request, user_login)
            if user_login.student.prefix == 'S':
                return render(request, 's.html')
            if user_login.teacher.prefix == 'T':
                return render(request, 't.html')
            if user_login.parent.prefix == 'P':
                return render(request, 'p.html')
        else:
            return render(request, 'index.html', {'error': True})
    else:
        return render(request, 'index.html')

System check identified no issues (0 silenced).
May 20, 2017 - 21:43:51
Django version 1.11.1, using settings 'e_diary.settings'
Starting development server at http://127.0.0.1:8000/
Quit the server with CTRL-BREAK.
[20/May/2017 21:43:56] "GET / HTTP/1.1" 200 983
Internal Server Error: /sign_in/
Traceback (most recent call last):
  File "C:\Users\USER\AppData\Local\Programs\Python\Python36-32\lib\site-packages\django\core\handlers\exception.py", line 41, in inner
    response = get_response(request)
  File "C:\Users\USER\AppData\Local\Programs\Python\Python36-32\lib\site-packages\django\core\handlers\base.py", line 187, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "C:\Users\USER\AppData\Local\Programs\Python\Python36-32\lib\site-packages\django\core\handlers\base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "C:\Users\USER\Desktop\e_diary\electronic_diary\views.py", line 29, in sign_in
    if user_login.teacher.prefix == 'T':
  File "C:\Users\USER\AppData\Local\Programs\Python\Python36-32\lib\site-packages\django\db\models\fields\related_descriptors.py", line 406, in __get__
    self.related.get_accessor_name()
django.db.models.fields.related_descriptors.RelatedObjectDoesNotExist: User has no teacher.
[20/May/2017 21:44:02] "POST /sign_in/ HTTP/1.1" 500 70872