My first post was just about the legalities, now I’ll discuss the practicalities. I don’t believe that a teacher smacking a child today with any regularity has any justification. (Would you allow a math teacher to smack a child? What is the difference.) Children today understand that there are many teaching styles and will view a teacher that smacks as a teacher that cannot control the students and resorts to violence. Regardless of the truth, a teacher that does not have the respect of the students cannot be an effective teacher.
If smacking were limited to extreme instances (not more than once per semester) and clearly demonstrated that the teacher was in full control a case can be made for its justification.