list index out of range

[DrPengin]

I am in the second half term of the first year of a GCSE Python course. For a piece of Homework, I need to write a program that converts marks to grades based on given grade boundaries, then store them in a list to print out the number and percentage of passes and fails, with grade "C" or better being a pass. here is what I have written:

def Grade(Mark):
    if 0 <= Mark <= 39:
        Grade = "U"
    elif 40 <= Mark <= 49:
        Grade = "E"
    elif 50 <= Mark <= 59:
        Grade = "D"
    elif 60 <= Mark <= 69:
        Grade = "C"
    elif 70 <= Mark <= 79:
        Grade = "B"
    elif 80 <= Mark <= 89:
        Grade = "A"
    elif 90 <= Mark <= 100:
        Grade = "A*"   
    return Grade

def FailPass(Count):
    NumPass = 0
    NumFail = 0
    for Loop in range(Count+1):
        if Grades[Count] not in("C","B","A","A*"):
            NumFail += 1
        else:
            NumPass += 1
    PerPass = (NumPass/Count)*100
    ParFail = (NumFail/Count)*100
    
    return [PerPass,PerFail,NumPass,NumFail]

Grades = []
Counter = 0
Input = 1
while Input != 0:
    Input = int(input("\nWelcome!\n\
Press '1' to convert a mark into a grade\n\
Press '2' to display the amount and percentage of passes\n\
Press '3' to display the amount and percentage of fails\n\
Press '0' to quit\n\
>>> "))
    if Input == 1:
        MarkIn = int(input("\nPlease enter the mark you wish to convert\n\
>>> "))
        if 0 <= MarkIn <= 100:
            GradeOut = Grade(MarkIn)
            print("\nA mark of",MarkIn,"is equivalent to a grade",GradeOut,)
            Grades.append(GradeOut)
            Counter += 1
            
        else:
            print("\nPlease enter a valid mark.")

    elif Input == 2 or Input == 3:
        FP = FailPass(Counter)
        if Input == 2:
            print("\nThere were",FP[3],"passes, which means %"+str(FP[1]),"of all students passed.")
        else:
            print("\nThere were",FP[4],"fails, which means %"+str(FP[2]),"of all students failed.")

but I get the error 'list index out of range' in line 22. I don't know what I am doing wrong. Also any other tips would be appreciated.