harry potter - Are Hogwarts teachers allowed to hit students?

In the movies we can see Snape smacking Harry and Ron several times. Is this acceptable?

Answer

In general it is not allowed but finally it comes down to who is running the school.

Dumbledore was against it:

Professor Umbridge seized Marietta, pulled her round to face her and began shaking her very hard. A split second later Dumbledore was on his feet, his wand raised; Kingsley started forwards and Umbridge leapt back from Marietta, waving her hands in the air as though they had been burned.

"I cannot allow you to manhandle my students, Dolores," said Dumbledore

_{^{Harry Potter and the Order of the Phoenix, Chapter 27, The Centaur and the Sneak}}

Professor McGonagall was against it:

“Teach - Moody, is that a student?” shrieked Professor McGonagall, the books spilling out of her arms.

“Yep,” said Moody.

“No!” cried Professor McGonagall, running down the stairs and pulling out her wand; a moment later, with a loud snapping noise, Draco Malfoy had reappeared, lying in a heap on the floor with his sleek blond hair all over his now brilliantly pink face. He got to his feet, wincing.

“We give detentions, Moody! Or speak to the offender's Head of House!”

_{^{Harry Potter and The Goblet of Fire, Chapter 13, Mad Eye Moody}}

_{^{We also see that during Dumbledore's and Professor McGonagall's absence, things were lot different.}}

_{^{Umbridge was planning to pass Educational Decree Twenty-nine, which allowed Filch to punish students.}}

_{^{'Yerse… I've been telling Dumbledore for years and years he's too soft with you all,' said Filch, chuckling nastily. 'You filthy little beasts would never have dropped Stink Pellets if you'd “known I had it in my power to whip you raw, would you, now? Nobody would have thought of throwing Fanged Frisbees down the corridors if I could've strung you up by the ankles in my office, would they? But when Educational Decree Number Twenty-nine comes in, Potter I'll be allowed to do them things…'}}

_{^{Harry Potter and the Order of the Phoenix, Chapter 28, Snape's Worst Memory}}

This also indicates that corporal punishments were not allowed. Special decree had to be passed.

There was "Approval for Whipping" form, the origin of this form is not very clear.

"Approval for Whipping… Approval for Whipping… I can do it at last… they've had it coming to them for years…"

He pulled out a piece of parchment, kissed it, then shuffled rapidly back out of the door, clutching it to his chest.

Filch elbowed his way closer to Umbridge, almost crying with happiness.

"I've got the form, Headmistress," he said hoarsely, waving the piece of parchment Harry had just seen him take from her desk. "I've got the form and I've got the whips waiting… oh, let me do it now…"

Harry Potter and the Order of the Phoenix, Chapter 29, Career Advice

Another instance of rule change:

"Nah, they make her look tame. The other teachers are all supposed to refer us to the Carrows if we do anything wrong. They don’t, though, if they can avoid it. You can tell they all hate them as much as we do."

"Amycus, the bloke, he teaches what used to be Defense Against the Dark Arts, except now it’s just the Dark Arts. We’re supposed to practice the Cruciatus Curse on people who’ve earned detentions –"

Harry Potter and the Deathly Hallows, Chapter 29, The Lost Diadem

It all comes down to who is running the school, because that person can modify the school rules.