C., Somehow your son has figured out that hitting gets a reaction, and it has worked, again.
The only solution at this point is to set limits with him, make it clear that he is to listen to you, and that hitting is not allowed. If he hits, do not respond with hitting him (spanking). Instead, the consequence should be a time out (one minute per year of age) in an area without toys but where you can clearly see him and ensure he SITS in time out.
I would also recommend if this is not successful that you enforce consequences such as removal of television priveleges, removal of a favorite activity (video game, toy, etc). If these fail, seek professional help through your pediatrician. Hitting can be an expression of more serious problems such as autism or Oppositional Defiant Disorder.
Hope this helps!
And a little PS to Melanie, of course this child has a right to education, but the teacher has a right to teach without any form of violence or threat. Hitting is unacceptable and should not be tolerated by any teacher.
V. Pickett, RN