A little context and better formatting clarifies all:
“Shame,” said Ron, helping himself to a jam doughnut. “He was starting
to grow on me.”
The rest of the final term passed in a haze of blazing sunshine.
Hogwarts was back to normal with only a few, small differences—Defense
Against the Dark Arts classes were canceled (“but we've had plenty of
practice at that anyway,” Ron told a disgruntled Hermione) and Lucius
Malfoy had been sacked as a school governor. Draco was no longer
strutting around the school as though he owned the place. On the
contrary, he looked resentful and sulky. On the other hand, Ginny
Weasley was perfectly happy again.
Too soon, it was time for the journey home on the Hogwarts Express.
Harry, Ron, Hermione, Fred, George, and Ginny got a compartment to
themselves. They made the most of the last few hours in which they
were allowed to do magic before the holidays. They played Exploding
Snap, set off the very last of Fred and George's Filibuster fireworks,
and practiced disarming each other by magic. Harry was getting very
good at it.
They were almost at King's Cross when Harry remembered something.
"Ginny--what did you see Percy doing, that he didn't want you to tell
"Oh that," said Ginny, giggling. "Well--Percy's got a girlfriend."
Fred dropped a stack of books on George's head. "What?"
"It's that Ravenclaw prefect, Penelope Clearwater," said Ginny.
"That's who he was writing to all last summer. He's been meeting her
all over the school in secret. I walked in on them kissing in an empty
classroom one day. He was so upset when she was--you know--attacked.
You won't tease him, will you?" she added anxiously.
"Wouldn't dream of it," said Fred, who was looking like his birthday
had come early.
"Definitely not," said George, sniggering.
Clearly they are on the train heading back to London and they are probably approaching or perhaps just inside London itself. If not "gathering their luggage" as you say, they are probably doing some last minute packing and rearranging of all the stuff that naturally accrues in one's cabin during a long train ride.
Obviously Fred is dealing with a stack of books, possibly in the process of shoving them into a trunk or satchel up in the rack above George's seat. It is Fred who says "what?"