Django Queryset for Forms not working

I don't know if I am trying the impossible but I have HTML select option that comes from Django's forms.Form's queryset, States.objects.all().

Model:

class Countries(models.Model):
    name = models.CharField(max_length=25)

Model:

class States(models.Model):
    country_id = models.ForeignKey('Countries')
    name = models.CharField(max_length=25)

Form:

class sign_up_form_school(forms.Form):
    states = forms.ModelChoiceField(
        queryset = States.objects.all(), 
        widget=forms.Select(attrs={
          'class': states.country_id.name #is this POSSIBLE?
          }))

I want each select option value to have different class as I have tried above but it returns error: name 'states' is not defined.