In my experience, all children do not benefit by physical punishment, at home or in school. One size does not fit all. However, there are some children who can benefit by controlled physical stimulus, if done in a rationally consistent manner. You set the rules and define the consequences so everyone knows reliable cause and affect.
For example, say you have a child with a very small attention span. They tends to act out and/or react on impulse. Since that child is changing their focus, often and fast, you will need a fast discipline procedure, that can be implemented to act within the small time frame of their focus. The quick slap; shock, is time proven way to help. A quick slap can be accomplished in a split second, while the child are in focus; allowing them to associate real time cause and affect. The time out approach can become too long, if their focus is going in and out. They may internally try to figure out, which focal point, out of many, is the problem?
The extra benefit of quick and direct assignment of focal cause and affect, is it does not waste all the teacher's time on a few problem children, due to techniques that often miss fast moving targets. This does an injustice to the rest of the students, who now lose their allotted instruction time. This change of discipline approach could explain lowering test scores. All children are important, and if the brats get all the attention, the lesson to the rest of the students, is to become more like brat, so you can get special treatment. One may end up with a classroom of chaos. This result is a form of psychological child abuse.
One observation, from political discussions with the Left Wing of culture, is this side of the political spectrum has more special needs; need fads, that they think overrides everyone else, with a Big Brother urgency. The drugs and the time out, did not do a good job, other than create a superficial bandaid. The impulsive behavior is now part of the adults. Your experts are fools.
On the other hand, if you have children who have self control, who can see cause and affect and focus for hours on their hobbies and projects, they do not need split second slap reinforcement. The slap can be very counter productive since it actually destroys their long term concentration and focus, with an anticipated fear, that may not even have a casual pattern. This is the down side of the slap. You need to tailor to both student types.
Micro managing children who have good focus, for example, can be detrimental to these healthy children. Now the adult, with the attention disorder, is leading the child who does not have it. That teacher may need the slap, since they can't help narrowing their focus onto minutia, away from the longer term picture needed for research. This type of teacher will have a harder time implementing a longer term approach of tailored slap discipline, in a rational way. That type of teacher may have missed slap therapy, as a child, and becomes abusive as an adult, thereby giving slaps a bad name. They may slap on impulse; bad day, all without logical reason.