Cailin,
Teachers should never smack. If you are a teacher, I am sure you know that a teacher/child relationship is completely different to a parent/child relationship. It is interesting that although most of the nursery teachers at our son's small pre-school nursery are theoretically anti smacking, most will admit having smacked their own children at some point or another and will admit that, at the time, they felt it was the most effective thing to do (although they then say they feel it was a failure of good parenting).
I think the difference is that, as a parent, you know your child so well and can focus on them so much that you KNOW whether what they are doing is naughtiness or something else. As a teacher, I think you would have to, on some level, guess. You cannot know what every child in your class is thinking.
From my memory of being a child, the only smacks I really remember and still resent are the ones where I was accidentally naughty because I did not understand what was wanted of me. The ones I was given where I just pushed and pushed until I got my smack were cathartic. I knew my mother (and it was mainly her, my father tended to smack when he lost his temper, which was rare and never right, so I never pushed him) cared enough to draw the boundary that I was actively asking her to draw. I was bright enough to not get smacked after about 4 or 5 as I worked out that I was always going to lose the battle. So, to those who say that you have to keep smacking, you don't. The lesson is learned.
To be honest, I think my wife and I smack ineffectively, in that our smacks are really not much of a deterrent. However, I find that "time outs" are next to useless. I get a chorus of "can I come in Daddy" for 3 minutes (he is 3) followed by a quick apology and then the behaviour is repeated within 24 hours. And I cannot bring myself to label my child naughty and use a naughty step (and I do think that is what a naughty step implies). I think that is possibly the worst punishment as once a child thinks of himself as naughty, it is hard to turn it around. Positive reinforcement does work, albeit terribly slowly. I think all good parenting revolves around effective communication and modelling behaviour. However, boundaries do have to be drawn and a smack is one way of drawing them. For those who say why do it if there are better alternatives, it is because I am not convinced the other alternatives are better, it is one tool of many within a parenting toolkit and sometimes the most effective. For example, trying to keep a 3 year old out while his 18 month old brother is at the other end of the room throwing soil out of a plant pot is not feasible. It is clearly not an option for those people who have a temper and where a smack can end up causing more than a few seconds of pain.