There is a presumption in your question that deserves further inquiry. Your son was "smacked" by a teacher. This covers a lot of ground and not all of it is necessarily inappropriate. Smacking could suggest a teacher giving "a high 5" as well as lots of other contact which should be no cause for concern. However, if by smacking you are suggesting the application of corporal punishment, clearly a boundary has been grossly crossed.
A teacher or school employee is prohibited from striking a student as a form of discipline. In Maryland, a teacher can be charged, among other things, with §3-203; Assault in the Second Degree. This carries a maximum punishment of up to ten years incarceration and/or a fine up to $2,500.00.
However, under specific circumstances a teacher may apply a measured physical response to a child’s physical aggression. School personnel, which include teachers, can use physical contact as part of a “continuum of force” as needed to respond, prevent or stop harm to other children and school employees.
Continuum of force means that a teacher may use only as much force as minimally required to bring control in a particular situation. The gym teacher using a baseball bat to break up a student fight in the locker room would be a no-no; grabbing a kid and pulling him off another however is a good application of appropriate force.
Therefore, to evaluate your child’s situation you will need to know whether your son was committing an act of physical aggression; and if yes, whether the response by the teacher was appropriate under the circumstances.
My recommendation to any parent who believes that there is a problem between their child and a teacher is to meet with that teacher and school principal with an open mind. Get the full facts of an event. It is important to recognize that a child’s perspective may be somewhat titled.
Teachers today are trying their best to educate our children and students always do better when parents and teachers are working together.
A growing concern, particularly in light of the tragic events of Newtown, Connecticut is whether teachers should be armed with handguns. In other words there are proponents to the idea that a permissive continuum of force should include the option of deadly force. This is a sobering thought and thankfully well beyond the scope of your question.