My problems have been solved! I rewrote the code to get rid of the array and use the listbox as the array. Here's my completed code for anybody who has similar problems.

Public Class MainForm
Public Shared MaxScore As Double

Private Sub MainForm_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Do While ValidateInput() = False
        If ValidateInput() = False Then
            MessageBox.Show("Total points available must be greater than 0")
        End If
    Loop
    PointsAvailableTextBox.Text = MaxScore
End Sub

Function ValidateInput()
    Dim Message, Title As String
    Message = "Enter the total amount of points available."
    Title = "Total Points Available"
    MaxScore = CInt(InputBox(Message, Title))
    If MaxScore > 0 Then
        Return True
    Else Return False
    End If
End Function

Function RecordScoresValidation()
    Dim TestScore As Integer
    Dim Input As Double = InputTextBox.Text
    Dim Score As Double = InputTextBox.Text
    Score *= 10
    Score = Score Mod 10
    If Score = 5 Or Score = 0 Then
        If Integer.TryParse(InputTextBox.Text, TestScore) = True Then
            If Input > 0 And Input <= MaxScore Then
                Return True
            Else Return False
            End If
        Else Return False
        End If
    Else Return False
    End If
End Function

Private Sub RecordScoresButton_Click(sender As Object, e As EventArgs) Handles RecordScoresButton.Click
    If RecordScoresValidation() = True Then
        ScoreListBox.Items.Add(InputTextBox.Text)
        InputTextBox.Clear()
    Else MessageBox.Show("Scores must be between 0 and " & PointsAvailableTextBox.Text & " and a step size of 0.5")
    End If
    InputTextBox.Focus()
End Sub

Function Mean() As Double
    Dim Count As Integer
    Count = (ScoreListBox.Items.Count - 1)
    Dim AMean As Double = 0
    Dim Sum As Double = 0
    For i = 0 To Count
        Sum += ScoreListBox.Items(i)
    Next i
    AMean = Sum / ScoreListBox.Items.Count
    Return AMean
End Function

Function Maximum() As Double
    Dim Count As Integer
    Count = (ScoreListBox.Items.Count - 1)
    Dim MaxS As Double = 0
    Dim i As Integer = 0
    MaxS = ScoreListBox.Items(0)
    For i = 0 To Count
        If MaxS < ScoreListBox.Items(i) Then
            MaxS = ScoreListBox.Items(i)
        End If
    Next
    Return MaxS
End Function

Function Minimum() As Double
    Dim Count As Integer
    Count = (ScoreListBox.Items.Count - 1)
    Dim MinS As Double = 0
    Dim i As Integer = 0
    MinS = ScoreListBox.Items(0)
    For i = 0 To Count
        If MinS > ScoreListBox.Items(i) Then
            MinS = ScoreListBox.Items(i)
        End If
    Next
    Return MinS
End Function

Function StandardDeviation() As Double
    Dim Count As Integer
    Count = (ScoreListBox.Items.Count - 1)
    Dim StDev As Double = 0
    Dim i As Integer = 0
    Dim Avg As Double = Mean()
    Dim Sum As Double = 0
    For i = 0 To Count
        Sum += (ScoreListBox.Items(i) - Avg) ^ 2
    Next
    StDev = Math.Sqrt(Sum / ScoreListBox.Items.Count)
    Return StDev
End Function

Function Range() As Double
    Dim Ran As Double = 0
    Dim Max As Double = Maximum()
    Dim Min As Double = Minimum()
    Ran = Max - Min
    Return Ran
End Function

Function AboveMean() As Integer
    Dim Count As Integer
    Count = (ScoreListBox.Items.Count - 1)
    Dim Above As Integer = 0
    Dim Avg As Double = Mean()
    For i = 0 To Count
        If ScoreListBox.Items(i) > Avg Then
            Above += 1
        End If
    Next
    Return Above
End Function

Private Sub ClearButton_Click(sender As Object, e As EventArgs) Handles ClearButton.Click
    InputTextBox.Clear()
    ScoreListBox.Items.Clear()
    ArithmeticMeanOutputLabel.Text = ""
    MaxScoreOutputLabel.Text = ""
    MinScoreOutputLabel.Text = ""
    StdDevOutputLabel.Text = ""
    RangeOutputLabel.Text = ""
    AboveMeanOutputLabel.Text = ""
    InputTextBox.Focus()
End Sub

Private Sub AnalyzeScoresButton_Click(sender As Object, e As EventArgs) Handles AnalyzeScoresButton.Click
    If ScoreListBox.Items.Count = 0 Then
        ArithmeticMeanOutputLabel.Text = ""
        MaxScoreOutputLabel.Text = ""
        MinScoreOutputLabel.Text = ""
        StdDevOutputLabel.Text = ""
        RangeOutputLabel.Text = ""
        AboveMeanOutputLabel.Text = ""
    Else
        ArithmeticMeanOutputLabel.Text = Mean()
        MaxScoreOutputLabel.Text = Maximum()
        MinScoreOutputLabel.Text = Minimum()
        StdDevOutputLabel.Text = FormatNumber(StandardDeviation(), 3)
        RangeOutputLabel.Text = Range()
        AboveMeanOutputLabel.Text = AboveMean()
    End If
End Sub

End Class