class StudentModel(models.Model):
    user = models.OneToOneField(User, unique=True)
    birth_date = models.DateField()
    contact_number = models.IntegerField()
    referral = models.CharField(max_length=100, choices=referral_choices)

from django import forms
from opus_login.models import StudentModel, EmployerModel

class StudentForm(forms.ModelForm):

    class Meta:
        model = StudentModel
        fields = ['username', 'first_name']

django.core.exceptions.FieldError: Unknown field(s) (username, first_name) specified for StudentModel

from django import forms
from opus_login.models import StudentModel, EmployerModel

class StudentForm(forms.ModelForm):
    username = forms.CharField()
    first_name = forms.CharField()

    class Meta:
        model = StudentModel
        fields = ['__all__']

    def save(self, **kwargs):
        student = super().save(commit=False)
        user = User.objects.create(username=self.cleaned_data['username'], first_name=self.cleaned_data['first_name'])
        user.set_password(self.cleaned_data['password']) #if there is a password field
        student.user = user
        student.save(commit=True)
        return student

class UserForm(forms.ModelForm):
    password = forms.CharField(label='Password',widget=forms.PasswordInput)
    password2 = forms.CharField(label='Repeat password',widget=forms.PasswordInput)

    class Meta:
    model = User
    fields = ('username', 'first_name')

    def clean_password2(self):
        .......
        return password2

class StudentForm(forms.ModelForm):
    class Meta:
        model = StudentModel
        fields = ['birthdate', 'contact_number']

def register(request):
    if request.method == 'POST':
        user_form = UserForm(request.POST)
        student_form = StudentForm(request.POST)
        if user_form.is_valid() and student_form.is_valid():
            user_form.save()
            student_form.save()

<form action="." method="post">
  {{ user_form.as_p }}
  {{ student_form.as_p }}
  {% csrf_token %}
  <p><input type="submit" value="Register"></p>
</form>