[Django]-Why does Google ReCaptcha not prevent the submission of the form?

I solved the problem myself. The NoReCaptchaField should not be in the Meta class. It worked like this:

class PostForm(forms.ModelForm):
    class Meta:
        model = Post
        fields = '__all__'
    captcha = NoReCaptchaField()

No I get an error message if the captcha is ignored, but that is a different question I guess.