Yeh, it definately sounds like windows messenger, I had to walk Imperial through how to disable it on windows XP.

Disabling Windows Messenger:
Open My Computer
Open Control Panel
Open Administrative Tools
Open Services
Double Click 'Messenger'
Click 'Stop'
From the dropdown choose 'Disabled'
Press Apply then Ok
Exit all screen's still open.

Now no more messages, it's not too hard to stop them, in my experiance with the messenger service is that it's completely pointless and shouldn't have been shipped turned ON since people use it to advertise on Broadband networks; or in your case to message stupid kids in a computer class...
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