K. -
I've written to other moms re: bullying, as I've been doing a LOT of research lately on this and other school / safety topics. If interested, feel free to view some of my other responses on this site. But first - GOOD for you for staying on top of this! We need more parents willing to get in and help problem-solve, not less!
My first concern is that you avoid 'mommy melt downs' like the plague - this is something that is not going to help you get what you want for your son. It's the first thing we moms want to do, by instinct - protect and defend - but there are ways to do this that will get you better results, much faster.
The fact is, the school SHOULD be on your side, as the law certainly is... The school has every reason to work with you, not against you (and if you go in, guns-a-blazing, you won't be soliciting this mutual interest). This does NOT mean you have to be 'fluffy' or sweet or a pushover - you can be firm, clear and direct without losing it, though. Here's a strategy I would try, if it were me:
I would first get all my facts on paper - what happened, when, where, who saw it (what their statements are) and who didn't (to your knowledge based on lack of phone call, etc.) - a whole timeline, clearly and concisely written. I would jot down my top three concerns, top three things I want taken care of, on behalf of my child. I would also write down my top three ideas as to how to resolve this situation - it can't hurt to bring ideas for solutions to the table, in addition to concerns. Ultimately, a list like this is for YOU, first and foremost - so you can focus on being a calm, assertive ADVOCATE, on behalf of your son, in a meeting with the principal, teacher, substitute, etc.
The other thing I would do is educate myself a bit about a) bullying and b) the school's perspective.
With regard to bullying, it is a growing problem in our nation's schools. And it's a problem lawmakers are on - schools are now required to have a ZERO TOLERANCE for bullying. Plainly, if bullying is happening, it's a threat to the safety of your child and to their learning (when children feel unsafe, it interferes with their ability to focus on education).
I strongly suggest picking up a copy of a book called Safe At School; Awareness and Action for Parents of Kids Grades K-12 by Carol Silverman Saunders.
I also suggest you go to sites like the National Crime Prevention Council and take a look at their .pdf on bullying (there are LOTS of free publications on their website that most of us don't realize we can get - VERY helpful stuff for kids and parents, alike - they're at www.ncpc.org). In their calendar publication, each month is dedicated to a subject - March, ironically, is dedicated to "Addressing the Problem of Bullying" - in this feature, they say, "Being bullied is not just an unpleasant rite of passsage through childhood. Research shows that people who were bullied as children are more likely to suffer from depression and low self-esteem, and bullies themselves are more likely to engage in criminal behavior later in life. Bullying can be physical (e.g., hitting, k8icking, or pushing), verbal (e.g., taunting, teasing, or name-calling), or psychological (e.g., spreading rumors or social exclusion)..." - people don't often think of gossip / rumors as bullying, but it's interesting to note. Anyway, further down, it states, "School bullying has come to the public's attention amid reports that it may have been a contributing factor in recent school shootings, and bullying behavior has been linked to other forms of antisocial behavior such as vandalism, shoplifting, skipping or dropping out of school, and the use of drugs and alcohol."
CLEARLY - it is not in your son's interest, the school's interest or the community's interest to have this activity going on in school (in kindergarten, no less!). Again, kids can't learn if they're stressed out - and if you want to see how the U.S. stacks up in education (compared to 24 other nations), take a look at this video - it's eye-opening - http://youtube.com/watch?v=kLBZqrzZvk8 .
As for the school's perspective, it's very important for you, as an advocate for your son, to appreciate (my perspective) that we live in a fear-based culture these days. It's understandable that schools may be on the defensive - people are so lawsuit-happy in this country and schools are left with every problem under the sun while being underfunded and lacking in support of fabulous parents (not all of us are in the classroom volunteering, etc. - I'm not perfect in this respect, either!). The fact is, volunteers and PTA simply cannot bring everything 'right' - the schools need ALL parents to come together, in the spirit of improving the state of things (and if you want to see what things are like in Washington's system, I urge you to see the recent Seattle PI feature - a study funded by the Bill & Melinda Gates Foundation - that's at http://seattlepi.nwsource.com/opinion/353294_focus02.html).
If your son is to 'win' here, you need to 'disarm' that principal. I would apologize for my meltdown and inform him of where I'd like to take this meeting - to let him know that my only interest is in ensuring my son's safety - that I understand we cannot change what HAS HAPPENED (let's all acknowledge the facts on my list and be open about this - a point I would insist on as a fair exchange for my apology, candor and best-foot-forward with them) but we can, together, certainly do something about this child who is posing a threat to the safety AND education of your child (if not others).
Ultimately, see if you can work WITH the principal and teachers, to achieve a plan that will ensure a win-win-win - that will allow your son a safe learning environment and get this boy the help he needs and ensure that the ZERO tolerance is being maintained, on behalf of all children and the school, as well.
K. - YOU are a super mom for following up on this and, if you take the time to consider all the information and create as strategy for yourself, you'll likely wind up helping more children than your son - and the school, too. That will feel really good. I wish you the VERY BEST in your efforts!