Assigning multiple Letter Grades to a list Box?

So in this program I am writing, I am writing a function that takes the average I have already calculated for each student,(StudentAvg1, StudentAvg2, etc..) assigning a letter grade to their average, then displaying in a list box. The grades are as follows:
90-100 A

80-89 B

70-79 C

60-69 D

0-59 F

This is my code so far, keep in mind that I am VERY new to VB /> :

Private Function AssignGrade() As String
        If StudentAvg1 > 0 Then
            Select Case StudentAvg1
                Case 90 To 99
                    Return ("A")
                Case 80 To 89
                    Return ("B")
                Case 70 To 79
                    Return ("C")
                Case 60 To 69
                    Return ("D")
                Case 0 To 59
                    Return ("F")
            End Select
        Else
            MessageBox.Show("Error!")
        End If

Private Sub btnAverage_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAverage.Click
        lstDisplay.Items.Add(AssignGrade())

But this is only for one Student(StudentAvg1), how do code it so I can use all of the students (5 to be exact), and assign them letter grades? I have tried commas, and the AND property, but I am stumped! Thank you for any advice!