Solution: Inheritance of "Student" and integration into the program

User.py

Student inherits from User.

class User:                                                 # superclass "User"
    def __init__(self, surname, firstname, dateOfBirth):    # constructor 
        self._surname = surname                             # protected properties/attributes => only one "_"
        self._firstname = firstname
        self._password = firstname[0] + surname[0] + dateOfBirth  
        self._dateOfBirth = dateOfBirth
        self._username = surname 
        
    def getSurname(self):                                  
        return self._surname
    
    def getFirstname(self):
        return self._firstname

    def getPassword(self):
        return self._password

    def getDateOfBirth(self):
        return self._password

    def getUsername(self):
        return self._password


class Student(User):    # subclass "Student" inherits every property and method from superclass "User"
    def __init__(self, surname, firstname, dateOfBirth, schoolClass):
        User.__init__(self, surname, firstname, dateOfBirth)  # constructor of superclass is invoked
        schoolClass.addStudent(self);   # adds himself to school class
        self.__schoolClass = schoolClass
    def getSchoolClass(self):              
        return self.__schoolClass

Be aware of the bold line in class Student's constructor: when a Student object is created he adds himself to SchoolClass object.

School.py

Only some refactorings in class SchoolClass regarding the new class Student have to be made:

class School:                           
    def __init__(self,schoolClasses):
        self.__schoolClasses = schoolClasses
        
    def getSchoolClasses(self):
        return self.__schoolClasses

    def addSchoolClass(self, schoolClass):        # adds a school class to school
        self.__schoolClasses.append(schoolClass)
        
    
class SchoolClass:
    def __init__(self,name, students):
        self.__name = name
        self.__students = students                # list with students
        
    def getName(self):
        return self.__name

    def getStudents(self):
       return self.__students

    def addStudent(self, student):                # adds a student to school class 
        self.__students.append(student)

testStudent.py

In the main program 3 objects of class Student are created. As fourth parameter the school class is added. As shown above Student's constructor adds student to school class. In this file students no longer have to be assigned to a school class.

from School import *                        # Import classes
from User import *
        
schoolClass1 = SchoolClass("c1",[])         
schoolClass2 = SchoolClass("c2",[])

student1 = Student("Miller","Sam","22.11.2004",schoolClass1)  # additional argument of type SchoolClass
student2 = Student("Smith","Jake","13.02.2003",schoolClass1)
student3 = Student("Hampton","Oliver","11.01.2004",schoolClass2)

school = School([schoolClass1, schoolClass2]) 

schoolClasses = school.getSchoolClasses()  
for sc in schoolClasses:
    print(f"School class: {sc.getName()}") 
    Students = sc.getStudents()
    for student in Students:    # student now knows school class
        print(f"\t{student.getSurname()}, {student.getFirstname()} in school class {student.getSchoolClass().getName()}")

Output

School class: c1
    Miller, Sam in school class c1
    Smith, Jake in school class c1
School class: c2
    Hampton, Oliver in school class c2