The number of elements in Python's list increases on its own.

import datetime

Namelist=[ ]
Genderlist = [ ]
Birthdaylist = [ ]
Nationallist = [ ]

StudentList= [ ]
StudentGender= [ ]
StudentBirthday= [ ]
StudentNationality= [ ]


def datainput():
    print("Enter your Birthday")
    y1 = int(input("Enter the year of your birthday.":")
    m1 = int(input("Enter the month of your birthday.":")
    d1 = int(input("Enter the day of your birthday.":")
    dt1 = datetime.date (year=y1, month=m1, day=d1)
    dt2 = "{0:%Y /%m /%d}".format(dt1)
    Btd = str(dt2)
    print("")
    print("Your birthday is", Btd, "."")
    print("")
    return dt1,Btd

def EnterStudentInformation (LowerGradeLimit):
    Grade=int(input("Enter Your Grade:")
    if Grade<LowerGradeLimit:
        print("You can't enter this school.")
    else:
        Name = str(input("Enter Your Name.:")
        Gender=str(input("Enter Your Gender.":")
        dt1,Btd = datainput()
        Nationality=str(input("Enter Your Nationality.":")
        Namelist.append(Name)
        Genderlist.append (Gender)
        Birthdaylist.append(Btd)
        Nationallist.append (Nationality)
        print (Namelist)
        print(Genderlist)
        print(Birthdaylist)
        print (Nationallist)

    return Namelist, Genderlist, Birthdaylist, Nationallist

def Sortenrollinglist (Namelist, Genderlist, Birthdaylist, Nationallist):
    StudentList=sorted (Namelist)
    print(StudentList)
    for i in range(0,len(Namelist)): # Maybe this is the problem
        StudentGender.append (Genderlist [Namelist.index(StudentList[i])])
        StudentBirthday.append (Birthdaylist [Namelist.index(StudentList[i])])
        StudentNationality.append (Nationallist [Namelist.index(StudentList[i])])
    print(StudentGender)
    print(StudentBirthday)
    print(StudentNationality)

    return StudentList, StudentGender, StudentBirthday, StudentNationality

def CheckEnrollingList (Enrollinglimit, StudentList):
    NoofStudent=len (StudentList)
    if NoofStudent<Enrollinglimit:
        welcome=True
    else:
        welcome = False

    return welcome, NoofStudent

def Appologize():
    print("Sorry, but your enrolling is finished.")

def OutputList (StudentList, StudentGender, StudentBirthday, StudentNationality, NoofStudent):
    for jin range (NoofStudent):
        print(StudentList[j], "", StudentGender[j], "", StudentBirthday[j], "", StudentNationality[j])
        print("")


Enrollinglimit=int(input("Enter student limitation for enrolling school.":")
LowerGradeLimit=int(input("Enter lower grade limitation for enrolling school.:")
welcome=True
if welcome == True:
    while welcome == True:
        Namelist, Genderlist, Birthdaylist, Nationallist= EnterStudentInformation (LowerGradeLimit)
        StudentList, StudentGender, StudentBirthday, StudentNationality=Sortenrollinglist (Namelist, Genderlist, Birthdaylist, Nationallist)
        welcome, NoofStudent=CheckEnrollingList (Enrollinglimit, StudentList)
        OutputList (StudentList, StudentGender, StudentBirthday, StudentNationality, NoofStudent)
elif welcome==False:
    Appologize()
OutputList (StudentList, StudentGender, StudentBirthday, StudentNationality, NoofStudent)