There is absolutely no reason or excuse for a teacher to ever choke a child. I don't know what state you live in, but I would definitely check with your state legislation regarding corporal punishment. If you have an attorney friend in church, your neighborhood, or even through free counseling, talk with that person about your rights as a parent.
You may also check with the school for free counseling for your child since he suffers as a result of this treatment. It is your right to be served, although they may be hesitant if they think a law suit is coming. There are probably also other organizations in your area which provide inexpensive treatment...check with the school counselor for these contacts.
What state do you live in? Tennessee state law (where I live) provides for the following: "Corporal punishment allowed if imposed in reasonable manner; local board of education shall adopt rules and regulations to implement and control corporal punishment in its schools." It further defines the use of corporal punishment only in the event of "good cause, in order to maintain discipline and order."
The web address I have put in the Sources section below describes corporal punishment laws in all the states. You really need to talk with your child's principal and your district superintendent, if your child is in a public school.
Slapping a child should never be considered corporal punishment. Whether done by a teacher or other adult, even a parent, slapping is always abuse, in my opinion.