manytomany.py

from django.db import models

class Subject(models.Model):
    name = models.CharField(max_length=128)


class Student(models.Model):
    name = models.CharField(max_length=128)
    subjects = models.ManyToManyField(Subject, through='StudentSubject')


class StudentSubject(models.Model):
    student = models.ForeignKey(Student, on_delete=models.CASCADE)
    subject = models.ForeignKey(Subject, on_delete=models.CASCADE)
    marks = models.FloatField()