Moving on.
“And now, before we go to bed, let us sing the school song!” cried Dumbledore. Harry noticed that the other teachers’ smiles had become rather fixed.
Dumbledore gave his wand a little flick, as if he was trying to get a fly off the end, and a long golden ribbon flew out of it, which rose high above the tables and twisted itself, snakelike, into words.
“Everyone pick their favorite tune,” said Dumbledore, “and off we go!”
And the school bellowed:
“Hogwarts, Hogwarts, 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.”
Why does this minhag stop the year Harry comes to Hogwarts? Granted, Harry (and in effect, us) misses the sorting in Book 2, Book 3, and Book 6, but there is no mention of the song at any other time. And he was there at the END of the Opening feast for most of them.