I did replaced it with MessageBox.Show() but with no positive results. I traced the order of execution of events and I think this could be the culprit. I modified my methods and placed some Debug code. Here are my modifications:

    Public Function GetStudid(ByVal pTextBox As TextBox) As String
        'Shows a context menu and returns the STUDID of the selected student
        Dim cm As New ContextMenu
        Dim mnuItem As MenuItem
        For Each dr As DataRow In DS.Tables("students").Rows
            mnuItem = New MenuItem(dr.Item("studname"))
            AddHandler mnuItem.Click, AddressOf cmEventhandler
            cm.MenuItems.Add(mnuItem)
        Next
        Debug.WriteLine("BEFORE CONTEXTMENU DISPLAY")
        cm.Show(pTextBox, New Point(0, 0))
        Debug.WriteLine("AFTER CONTEXTMENU DISPLAY")
    End Function

    Private Sub cmEventhandler(ByVal sender As Object, ByVal e As EventArgs)
        Dim cm_index = CType(sender, MenuItem).Index
        studid = DS.Tables("students").Rows(cm_index).Item("studid")
        Debug.WriteLine("STUDID CHANGED")
    End Sub

DEBUG Output:
BEFORE CONTEXTMENU DISPLAY
AFTER CONTEXTMENU DISPLAY
STUDID CHANGED

What I am expecting is somthing like this:
BEFORE CONTEXTMENU DISPLAY
STUDID CHANGED
AFTER CONTEXTMENU DISPLAY

Could this problem be related to asynchronous execution?

thanks