Considering the attitudes in this thread, it's no wonder that a lot of children and teens nowadays are spoiled, ill-mannered, obnoxious brats with a sense of impunity and entitlement.
Speaking as an ex-teacher, I think the issue is that people too often conflate 'discipline' and 'corporal punishment'. Take my points with a grain of salt...as well as being an ex-teacher, I was a psychology major, which might make my opinions suspect to some...(
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Couple of points to my mind;
1) Parents absolutely need to take responsibility for disciplining their kids. This requires a large investment of effort, and time, and needs to be a priority. Else don't have kids. My experience as a parent is that positive reinforcement is more effective long term than negative, but I do use negative reinforcement as well. Kids who are EFFECTIVELY disciplined at home, amazingly, cause little trouble at school.
Some parents don't have the required skills to do this, but it seems more common that they don't make the required effort on a consistent basis.
2) Where 1 doesn't occur, whatever the school does is going to be of limited effectiveness. Brats be brats, and a teacher occasionally spanking them is unlikely in the extreme to promote any sort of ongoing positive behavioural change.
3) What spanking may do is promote resentment in a way that other negative reinforcements may not do. I'd admit it's hard to be 100% certain on this. It's very unlikely that spanking is LESS harmful though.
4) Positive reinforcements are consistently shown to be the most effective means of modifying behaviour on a long term basis.
5) Here's the kicker. They take time. Changing one kid's behaviour out of a class of 20 is actually quite feasible, even without home support (as long as the home environment isn't completely destructive to this). However, how much time can/should a teacher invest in modifying one kid's behaviour, especially if the parents have failed in step 1?