models.py

class Group(models.Model):

    students = models.ManyToManyField(Student, related_name='groups',
                                      through='StudentGroupMembership')

    @property
    def active_students(self):
        return Student.objects.filter(
            StudentGroupMembership.get_active_lookup())

    @property
    def active_teachers(self):
        return Teacher.objects.filter(
            GroupSubjectTeacher.get_active_lookup())



class Klass(Group):
    tutor = models.ForeignKey(Teacher)


class DateBoundMembership(models.Model):

    start = models.DateField()
    end = models.DateField(blank=True, null=True)

    @classmethod
    def get_active_lookup(self, date=None):
        ptr_field = self._meta.fields.get(self.MembershipMeta.member_ptr)
        k = lambda l: ptr_field.related_name + '__' + l
        if date is None:
            date = datetime.date.today()
        return Q(**{k('start__lte'): date}) & (
            Q(**{k('end__isnull'): True}) | Q(**{k('end__gte'): date})
        )

    class Meta:
        abstract = True


class StudentGroupMembership(DateBoundMembership):
    student = models.ForeignKey(Student, related_name='group_memberships')
    group = models.ForeignKey(Group, related_name='student_memberships')

    class MembershipMeta:
        member_ptr = 'student'
        group_ptr = 'group'


class GroupSubjectTeacher(make_date_bound_membership('teacher')):
    teacher = models.ForeignKey(Teacher)
    group = models.ForeignKey(Group)
    #subject = models.ForeignKey(Subject)