Don't work cleaned_data in Django

Question

This is my code:

class HighschoolForm(forms.ModelForm):

    class Meta:
        model = Highschool
        fields = ['id', 'dni', 'name', 'address', 'city', 'country', 'phone', 'mobile', 'mail', 'website', 'contact', 'entrydate']

    def clean_mail(self):
        mail = self.cleaned_data.get('mail')   #self.cleaned_data['mail']
        mail_base, proveedor = mail.split('@')
        dominio, extension = proveedor.split('.')
        if extension == 'ptn':
            raise forms.ValidationError('Does not allow Pluton mails....')
        return self.cleaned_data['mail']

However when I introduced data in ModelForm in the admin view a mail with a "ptn" extension, Django doesn't refused the data and record in the database. Which is the problem? I read de Django 2.0 documentation and I don't find the failure. Thanks