How to filter by field in ListView in Django?

Question

Django

There are models:

class School(models.Model):
    slug = models.SlugField(unique=True, verbose_name='ЧПУ')
    title = models.CharField(max_length=300, verbose_name='Название')

class Review(models.Model):
    school = models.ForeignKey(School, verbose_name='Школа')
    RATING_CHOICES = (
    (1, '1'),
    (2, '2'),
    (3, '3'),
    (4, '4'),
    (5, '5'),
    )
    rating = models.IntegerField(choices=RATING_CHOICES, default='5')

class SchoolList(ListView):
    model = School
    def get_context_data(self, **kwargs):
        context = super(SchoolList, self).get_context_data(**kwargs)
        context['review_count'] =  Review.objects.filter(school__slug=???).count().aggregate(Sum('rating')).values()[0] /Review.objects.filter(school__slug=???).count()
        return context

Displays a list of schools. Schools have reviews, so I want to display the count of reviews for each school in the list. I need to filter the Review by the schools in the list, I can't catch up on how to do it.