So you're saying the teachers were standing there watching this and saying nothing?
Or were they present but preoccupied with rounding up other children, finding lost things, answering a ton of questions, breaking up other altercations, talking to parents and making sure everyone was getting out ok?
My heart breaks for kids who are shy at first or have been treated this way, but I don't know any teachers who allow things like that to continue if they are aware of it. Please, you need to realize that while your child is your special angel, there are 15 special angels who also require attention, education, protection and provision at the same time which means it is not reasonable to expect their teachers to catch everything. You were waiting for your son, watching only him, so of course it stood out to you.
In addition, at this age, kids are learning how to get along with each other. Every little spat does not constitute bullying! They are learning to share space, work together and communicate appropriately. Shocking as it is, many children do not come to school with these skills already taught at home, so yes, there are things like this to work through. It's a LEARNING PROCESS for them and a more helpful approach would have been to address the girl and question her ("what are you doing? Is that a nice way to talk to someone?") as opposed to jumping to the conclusion that this is or would result in a serious bullying situation.
We need to teach our kids how to respectfully defend themselves when they encounter these opposite personalities. I know your son is young, but the sooner, the better! Neither they - or you - will be able to change or control everyone else who challenges them in their lives. Please start equipping your kids NOW! If you get in the habit of jumping in to resolve all of their problems, that can not only be a hard habit for YOU to break, but will ultimately rob them of the opportunity to take charge of their own business down the road.
Please be reasonable when you approach the teachers tomorrow. Being defensive and assuming the worst about them (that they don't care or are negligent) will not bode well for anyone. Get the full story before judging. Remember, they're on your side.
ETA: Cheryl, it is very unfortunate that you seem to think that MOST teachers only care about getting home or getting paid (our big fat paycheck - HAHAHA!) I know and work with hundreds of teachers and I can tell you that MOST of them are the exact opposite. In addition, most schools have a zero-tolerance bullying policy and require teachers to step in when they see bullying. Many kids, esp girls, don't do their bullying out in the open.