I found it a little odd that Professor McGonagall spent an entire day in Privet Drive on a weekday (Mr. Dursley goes to work) during school term (Nov. 1). So was Hogwarts closed at the time?
There are two aspects to this:
1) In the real world, 1st November 1981 was a Sunday. Maybe, just maybe, Vernon's job making drills was not a Mon-Fri, 9-5 job, though it certainly seems to be. Maybe Harry's parents died on Saturday night and the events described happen during Sunday, with Vernon going to work as some people do and McGonagall spending Sunday daytime on the wall.
2) However, as much as I love Ms Rowling's writing, she isn't great at the finer mathematical details - she says so herself! So she probably decided that this was a working week day without consulting a calendar.
Assuming that this is true: I think it very unlikely that Hogwarts was closed (although of course it was possible as it was during a very bleak part of a serious war) because a recent past closure was never mentioned when it was considered being closed in Chamber of Secrets. Indeed, the impression is given then that the illustrious, historic school has not had to close its doors for generations.
The suggestion throughout Chapter 1 of Philosopher's Stone is that the day, whatever day of the week it was, was given over to celebration.
When finding out what McGonagall has been up to, even Dumbledore is surprised she hasn't been celebrating.
All day? When you could have been celebrating? I must have passed a dozen feasts and parties on my way here.
Note that Mr Dursley during the day also walks into excited, barely-hiding wizards on the street in non-Muggle dress, and McGonagall complains about this fact as well as other issues being reported on the Muggle news. The adults are certainly celebrating, so it seems safe to assume that Hogwarts would at least have given the children a feast and a day off of lessons. This allows McGonagall to spend the day watching the Dursleys, even if Dumbledore himself didn't necessarily approve!
Also, considering Dumbledore is the kind of headteacher to cancel end of year exams as a celebration, I'm sure he'd give the children a standard term one day off to mark the (apparent) defeat of the worst Dark Wizard ever.