add tutoringgroup view to student api

oser_cs/tests/test_api/test_student_api.py

 """Student API tests."""
+from rest_framework import status
 from tests.factory import StudentFactory, TutoringGroupFactory, UserFactory
 from tests.test_api.mixins import ProfileEndpointsTestMixin
 from tests.utils.api import HyperlinkedAPITestCase
@@ -61,4 +62,11 @@ class StudentEndpointsTest(ProfileEndpointsTestMixin, HyperlinkedAPITestCase):
         return response
 
     def test_retrieve_tutoring_group(self):
-        pass  # TODO
+        def perform_retrieve_tutoring_group():
+            obj = self.factory.create()
+            response = self.client.get(
+                '/api/students/{}/tutoringgroup/'.format(obj.pk))
+            return response
+
+        self.assertRequiresAuth(perform_retrieve_tutoring_group,
+                                expected_status_code=status.HTTP_200_OK)

oser_cs/users/views.py

 """Users API views."""
 
 from django.contrib.auth import get_user_model
-from rest_framework import viewsets
-from rest_framework.mixins import (
-    ListModelMixin, RetrieveModelMixin, CreateModelMixin,
-)
 from dry_rest_permissions.generics import DRYPermissions
-from .models import Tutor, Student, SchoolStaffMember
-from .serializers import (
-    UserSerializer, UserCreateSerializer, UserUpdateSerializer,
-    TutorSerializer, StudentSerializer, SchoolStaffMembersSerializer,
-)
+from rest_framework import viewsets
+from rest_framework.decorators import detail_route
+from rest_framework.response import Response
+
+from tutoring.serializers import TutoringGroupSerializer
+
+from .models import SchoolStaffMember, Student, Tutor
+from .serializers import (SchoolStaffMembersSerializer, StudentSerializer,
+                          TutorSerializer, UserCreateSerializer,
+                          UserSerializer, UserUpdateSerializer)
 
 User = get_user_model()
 
@@ -104,6 +105,15 @@ class StudentViewSet(ProfileViewSet):
     queryset = Student.objects.all()
     serializer_class = StudentSerializer
 
+    @detail_route()
+    def tutoringgroup(self, request, pk=None):
+        """Retrieve the tutoring group of a student."""
+        student = self.get_object()
+        tutoring_group = student.tutoring_group
+        serializer = TutoringGroupSerializer(tutoring_group,
+                                             context={'request': request})
+        return Response(serializer.data)
+
 
 class SchoolStaffMemberViewSet(ProfileViewSet):
     """API endpoint that allows school staff members to be viewed or edited."""