What can be done to stop teenagers posting defamatory material about their teachers on the internet?

[RED-CHROME]

Defamation is defamation. Inform the Police. If the people doing that do not identify themselves then it is very unlikely anyone will take any notice of what they say. If, on the other hand they identify themselves, then go on them with the full strengh of the law. -- I would !!
If they are young, then the parents are ther to answer for them.
So you could fight back.
You could also pretend to be someone else and post other things just as bad about those kids but remain anonymous, though.... - Say things about those kids that will embarass them.
Finally, if it is being done on a large scale then do not worry about it -- other teahers are being victims of the same thing.