I love the Irish song, "The Parting Glass," but it probably isn't quite appropriate for a school girls' farewell. But my favorite farewell song is the last verse of the Scottish "Will Ye No Come Back Again."
Sweet the lav'rock's note and lang,
Liltin' wildly up the glen,
Och, to you we sing this song,
'Will ye no' come back again.'
As a young man, I lived and worked for some time at Tuolumne Co-Op farm in California. Several families lived on the farm and many visitors came for a day or week. Whenever visitors were leaving all of the residents would gather and send them on their by singing that stanza of the old Scottish song.