Get something to eat so you can think clearly. How your son sees you deal with this will set an important tone for the future. You can't be out of your mind upset or so angry you're looking at things from a retaliatory standpoint.
You need to try to calm down so that you can pursue further information rationally.
I definitely think you should schedule a meeting with the teacher and the school director. You need to voice your concerns, but you also have to be willing to have an open mind.
For instance, were there any marks on your son?
I'm not saying he's making it up, but it's interesting to me that he told you she slapped him, but instead of asking you to tell her to stop hitting him, he asked you to tell her to stop yelling at him.
Also, you need to consider what the definition of yelling is. Raising your voice to a level where everyone in the class can hear you is not the same as yelling in an attacking way. The teacher said she may have yelled a bit, let her explain what she meant by that....the circumstances.
We were at a bbq with friends and all kinds of kids running around. The hostess' son was 5. We were watching all the kids play in the yard when her son tripped and fell. He started crying and ran over to the group of kids and punched his sister. Long story short, he punched her because she pushed him down and hurt him. Forget the fact all the adults saw what happened and his sister was nowhere near him. He insisted she shoved him. He was sent to his room for hitting his sister, refusing to apologize to her and for making up a story.
It's happened more than once that a kid my son's gone to school with has gotten disciplined for something at school and then all the sudden a teacher threw a chair, grabbed a kid, hit a kid. Except my son and other kids in the class said it never happened.
In that event, it's a matter of a child having the attitude that "no one tells me what to do except my parents" and the stories begin to get the teacher into trouble because the kid knows the parents will go off. What the child did to deserve the discipline goes right out the window because the parents fly off in a different direction and the main issue never gets addressed.
I hate to say that little kids are capable of these things, but they are and it doesn't make them evil, but this is where a parent keeping a cool head and hearing all the facts before gathering the villagers and the pitch forks sends a strong signal to the child that mom will find out the best way to handle it.
I think it's highly likely that the teacher may have placed her hand on your son's back to guide him to his seat or to the play area or whatever.
I can guarantee you that if your son was slapped upside the back of the head, it's not something the other kids would have forgotten, IF it happened.
Just like a chair being thrown at a child. Not only is it a serious allegation, but the other kids would have been shocked or scared or mentioned it. But it didn't happen.
Take a deep breath, eat. Don't go off half-cocked. If your son knows he can get you this upset, this will no doubt be the last time and you don't want to begin on that foot when it comes to school.
The teacher may not be a good fit. Your son may not be ready for preschool just yet.
You can't make those decisions without a clear head and an open mind.
Best wishes.