"Farewell, Farewell" by Fairport Convention is a lovely tune but the lyrics might be a bit heavy for the occasion. Then there's always School's out for the Summer by Alice Cooper *grin*.
And then there's the one Joan Baez used to do I think it's called "10,000 Miles" it starts out "Fare thee well I must be gone and leave you for a while/ Wherever I go I will return if I go 10,000 miles..." A beautiful song.