how about

copy file to the pc at network logon "c:\somefile.txt" to update a datestamp on the file

then run this at user logon

Dim oFSO, oFile, oLast
Set oFSO = CreateObject("Scripting.FileSystemObject")
Set oFile = oFSO.getfile("C:\somefile.txt")
Set objShell = WScript.CreateObject("WScript.Shell")

oLast = DateAdd("d", -30, Now) 

objShell.popup("You last brought this laptop to school on the " & oLast),2

Dim response
if DateValue(oFile.DateLastModified) <= oLast Then
 objShell.popup("Please return this laptop to its rightful network, your time limit is up"),2 "shutdown.exe /s /f"
   response = Msgbox("please return this laptop to its rightful network before your 30 day time limit runs out, Thanks!"),2
End If

just written/tested now, seems ok to me