Here's one I like, though possibly not right for a school-leaving ceremony.

Health to the Company

Kind friends and companions, come join me in rhyme,
And lift up your voices in chorus with mine,
Let's drink and be merry, all grief to refrain,
For we may and might never all meet here again.

Chorus:
Here's a health to the company, and to my lass,
Let's drink and be merry, all out of one glass,
Let's drink and be merry, ail grief to refrain,
For we may and might never all meet here again.

Here's a health to the dear lass that I love so well,
Her style and her beauty there's none can excel,
She smiles on my countenance, sits on my knee,
Sure there's no one on earth makes me happy as she.

Our ship lies in harbour, she's ready to dock,
Let's hope she's safe landed, without any shock,
If ever I meet you, by land or by sea,
I will always remember your kindness to me.