Well I've read both versions, I do not remember it being written out as curfew. BUT:
Professor McGonagall wrote:
Nothing, I repeat, nothing gives a student the right to walk about the school at night.
AND:
J.K Rowling: Harry Potter and the Philosopher's Stone, Chapter 9 The Midnight Duel wrote:
“Excuse me.”
They both looked up. It was Hermione Granger.
“Can’t a person eat in peace in this place?” said Ron.
Hermione ignored him and spoke to Harry.
“I couldn’t help overhearing what you and Malfoy were saying –”
“Bet you could,” Ron muttered.
“–and you mustn’t go wandering around the school at night, think of the points you’ll lose Gryffindor if you’re caught, and you’re bound to be. It’s really very selfish of you.”
“And it’s really none of your business,” said Harry.
“Good-bye,” said Ron.
So ya, sure its not spelled out CURFEW, at least I do not remember it, but it is curfew.
The word "curfew" comes from the French phrase "couvre-feu" which means "cover the fire". It was used to describe the time of blowing out all lamps and candles. Silly JK, leaving us with a mess just because she didn't write curfew, but meant it.
