In Goblet of Fire Chapter 12 we see that Dennis Creevey starts at Hogwarts that year:
It was Colin Creevey, a third year to whom Harry was something of a hero.
“Hi, Colin,” said Harry warily.
“Harry, guess what? Guess what, Harry? My brother’s starting! My brother Dennis!”
By all accounts, then, in Order of the Phoenix (one year later) he should be in his second year. Yet in Chapter 16 of Order of the Phoenix we find him in Hogsmeade for the first D.A. meeting (my emphasis):
First came Neville with Dean and Lavender, who were closely followed by Parvati and Padma Patil with (Harry’s stomach did a back flip) Cho and one of her usually giggling girlfriends, then (on her own and looking so dreamy that she might have walked in by accident) Luna Lovegood; then Katie Bell, Alicia Spinnet, and Angelina Johnson, Colin and Dennis Creevey, Ernie Macmillan, Justin Finch-Fletchley, Hannah Abbott, and a Hufflepuff girl with a long plait down her back whose name Harry did not know; three Ravenclaw boys he was pretty sure were called Anthony Goldstein, Michael Corner, and Terry Boot; Ginny, followed by a tall skinny blond boy with an upturned nose whom Harry recognized vaguely as being a member of the Hufflepuff Quidditch team, and bringing up the rear, Fred and George Weasley with their friend Lee Jordan, all three of whom were carrying large paper bags crammed with Zonko’s merchandise.
We know from Prisoner of Azkaban Chapter 1 that Hogsemeade is only permitted to third years and above (my emphasis):
Dear Mr. Potter,
Please note that the new school year will begin on September the first. The Hogwarts Express will leave from King’s Cross station, platform nine and three- quarters, at eleven o’clock.
Third years are permitted to visit the village of Hogsmeade on certain
weekends. Please give the enclosed permission form to your parent or guardian to sign.
A list of books for next year is enclosed.
Professor M. McGonagall
We also know from Order of the Phoenix Chapter 16 that Filch was checking the permission status of everyone leaving the castle for that very Hogsmeade visit:
The morning of the Hogsmeade visit dawned bright but windy. After breakfast they queued up in front of Filch, who matched their names to the long list of students who had permission from their parents or guardian to visit the village.
How, then, was Dennis Creevey able to get into Hogsmeade, considering he would have only been a second year?