Against it mainly because there is a fine line between it and abuse/assault. and people do not know the difference, nor do they know how to fairly or SAFELY administer it. Some don't know the difference.

I had a teacher that used corporal punishment to good effect. He gave paddlings and he was preferably very formal about it - you actually had an appointment in the principal's office to get it. The wait was much worse than the two whacks with a ping-pong paddle. You would be made to bend over, grab your ankles, and WHACK WHACK. If he caught you doing something really bad, you might get a couple smacks on your ass with his hand on the spot which was terrifying but never injurious. He never injured anyone and was very fair with it (you really had to f-up to get the paddle). I got two from him, one with hand for being ejected from a class with a substitute, and one formal with the paddle for not doing homework (he had said when he gave the assignment anyone not doing it would get a paddling - I did. It made me think about consequences.

I had another teacher that beat the shit out of kids and would be in prison today for it (and should be). He had a wooden board with a handle for a paddle and I watched him call one big kid up in front of the class for making a little noise and beat him until the kid was wailing and could hardly walk back to his seat. Physical injury and mental humiliation. Terror. One of my younger siblings had that guy for a teacher a few years later and in that class that teacher smashed one of those little windows right out of a door with a kid's face. There was blood. This teacher punished only boys, never girls. Anything could set him off. He got away with it and was well though of. I was terrified of him.