Go to your Voice Mail on your desk or classroom phone.

Enter your password then press #.

Listen to your messages, and after you've listened to it, press 3 to delete.  

Do this for all voice mail messages.  

Now to permanently delete those already deleted messages (this is a safeguard in case you accidentally delete an important message):

Press 7 for Mailbox options.

Press 8 and then to confirm permanent deletion of VM messages, press 1.