"And now, before we go to bed, let us sing the school song! Everyone pick their favorite tune and off we go!"
-- Albus Dumbledore (PS7)
Dumbledore directs the school song at the end of the start-of-term feast when he feels in a particularly festive mood. The last time the song was known to be sung was during the feast on September 1, 1991 [Y11], when Harry Potter came to Hogwarts. Dumbledore directs the song with his wand. He flicks out a ribbon that floats in the air, creating the words for the students to sing along with, each singing to the tune of their choice. The rest of the staff does not appear to share Dumbledore's delight in this little ritual, but he conducts the singing with gusto and even gets a bit misty at the end of it (PS7).
The school song is not sung every year. In times of crisis, Dumbledore does not include the song in the opening festivities. With the rise of Voldemort in the mid-1990s [c. Y15], the song has not been sung recently (JKR).
Hogwarts School Song
Hoggy Warty Hogwarts,
Teach us something, please,
Whether we be old and bald
Or young with scabby knees,
Our heads could do with filling
With some interesting stuff,
For now they're bare and full of air,
Dead flies and bits of fluff,
So teach us things worth knowing,
Bring back what we've forgot,
Just do your best, we'll do the rest,
And learn until our brains all rot.