Public Class Form1

    Dim dblRunning, dblCurrentScore, dblHighest, dblLowest, dblAverage As Double
    Dim intCounter As Integer

    Private Sub btnScores_Click(sender As Object, e As EventArgs) Handles btnScores.Click
        Try
            'Initialized the variable just above limit
            dblLowest = 100.1
            While True
                dblCurrentScore = InputBox("Type your test score plus enter to submit it, or -1 plus enter to exit.", "Enter Scores")
                If dblCurrentScore = -1 Then
                    Exit While
                ElseIf dblCurrentScore > 100 OrElse dblCurrentScore < 0 Then
                    MessageBox.Show("Please enter a score between 0 and 100.", "ERROR!")
                Else
                    intCounter += 1
                    dblRunning += dblCurrentScore
                    If dblCurrentScore > dblHighest Then
                        dblHighest = dblCurrentScore
                    ElseIf dblCurrentScore < dblLowest Then
                        dblLowest = dblCurrentScore
                    End If
                    dblAverage = dblRunning / intCounter
                End If
            End While
        Catch ex As Exception
        End Try
    End Sub

    Private Sub btnStats_Click(sender As Object, e As EventArgs) Handles btnStats.Click
        lblOutput.Text = "Highest score: " & dblHighest & vbCrLf & "Lowest score: " & dblLowest & vbCrLf & "Average score: " & dblAverage
    End Sub
End Class