It is important to stay on top of the situation.  Since I don't know how old your child is, some of these suggestions may work better than others.

First, give your son a big hug.  He did the right thing in tell you that he is being bullied.  Keep talking to him.  He needs to know someone is on his side and fighting for him.

Most school districts have a anti-bulling policy.  Call the school and ask where you can find a copy of it.  If they are no help, then contact the school district.  There is probably some department called violence prevention or health services on the district level who can help you.
 
I would then call the school to set up an conference with both the administration and your son's teacher(s).  Inform them of the bullying situation.  Try not to get emotional, just state the facts (a timeline might help). Then ask lots of questions like:
What steps will the school take to make sure your son is safe?
What will happen to the bully?
Will the bullies parents be contact?
Do you need to fill out any forms reporting this situation?
How does the school implement the district's anti-bullying policy?
What can you do to become involved in making the school a safer place?
Do the teachers see the bullying towards your son taking place?
If so, what do they do about it?
 
If your son is attacked be sure the authorities are contacted and changes are filed.  Whether you go through with any legal action is up to you, but make sure you have a paper trail.  (This goes for the school decisions, too.)  You don't want it to come down to a he said, she said situation with nothing to back up your story.
 
Unfortunately, bullying goes beyond the school hours.  Cyberbullying often accompanies face-to-face bullying.  This can come in the form of phone calls, texting, emails, online slam books or pages, instant messaging, chat rooms, etc.  Don't be afraid to check your son's Facebook page and his friends' pages and text messages he has received as well as sent.
 
Ask your son what he thinks is motivating the bully to pick on him.  Did he innocently comment of something the bully said or did?  Keep the lines of communication open.  Reassure him that you are there for him and will do whatever you can to protect him.
 
I'm sorry the principal is being non-responsive.  If you still aren't getting any where with him, try putting everything in a letter and send it certified mail.  That way you have a receipt that says you've been trying to reach out to him.  

As for getting a speaker to come out, ask the school if they have a Just Say No Club.  If they do, contact the teacher who is running it.  He/she maybe of some help.

Keep after the school.   Education.com has fantastic resources you might find helpful.  http://www.education.com/topic/school-bullying-teasing/

Good luck!

Do you have a speaker in mind? If not I suggest Jodee Blanco; she is the author of "Please Stop Laughing at us" and also a victim of malicious bullying. i am a parent and currently a student of  investigative journalism.While writing a research paper on peer abuse/cyberbullying I stumbled upon Ms. Blanco and found her to be an inspirational speaker/author/survivor. I think that your situation would be right up her alley. perhaps if you contact her at the web site  provided and explain the issues with this stubborn and apathetic principle perhaps it may inspire her to provide a little assistance in this imperative mission. Personally, I would remind him of the most recent and disheartening occurrence of bully-cide involving that sweet  little South Hadley freshman Pheobe Prince who was bullied into an early grave.But also urging this man to keep in mind that although he may not be aware of or desire to acknowledge the severity of peer abuse (which in  statistical fact,produces permanent psychological damage and in many cases death),he may very well value his administrative position and could easily be humbled if his inattentive  behavior were to facilitate an incident resembling senseless and preventable tragedy in South Hadley. I would also subtly remind him of the embroilment afforded to Superintendent Gus Sayer for his negligence.In addition to every parental figure in South Hadley and abroad gunning for him like an angry pitchfork bearing mob, he was recently forced to resign.  If that approach don't work, reverse psychology can be a beautiful thing!!! try appealing to his ego a bit!!! fill his head with images of himself becoming the next great "pietist" of the academic world!..lol.Explain to him that school officials near and far would forever idolize his valiance and sing songs in honor of his heroic efforts to provide saftey and well-being to all the nation's defenseless little children oppressed by  "Goliaths" who imprison their schools with fear!!! In all seriousness though, I can no longer fetter my seething contempt for the growing negligence within our nation's academic structure and i must freely admit that i am completely sickened and disappointed in the lack of ratification displayed by "high horse" school officials.I am in the belief that some.. not all.. yet most,  have become too comfortable in their positions and have either lost or set aside the chivalrous duty to the children they once agreed to uphold.  many changes have need to be made not just concerning bullying.i think it is time for all of us as parents to do a little spring cleaning! out with the cold and in with the bold. Our children need responsive and motivated academic role models who aren't afraid to do what ever it takes to bring them a sense of security and protection as well as advocate justice when they are persecuted and harassed mercilessly by  predatory little heathens lurking in the hallways of their schools waiting to pounce on them.After all school officials are supposed to be public servants,they take an oath to educate and protect their students.long ago when i was an awkward prepubescent, my 2nd grade teacher used to refer to us as her "little chick-lings" and she was our protective mother hen teaching and nurturing us as if we were here own.What has happened people? Today's educational providers fail to implement "scholarly education"  but inadvertently  conducting a crash course in anti-trust/ insecurity 101!!! All the while having bold audacity to complain of juvenile delinquency, they cast accusatory slurs at the parents,and my personal favorite, suggestions "narcotizing" our children! Section I: Ethical responsibilities to children

Childhood is a unique and valuable stage in the human life cycle. Our paramount responsibility is to provide care and education in settings that are safe, healthy, nurturing, and responsive for each child. We are committed to supporting children's development and learning; respecting individual differences; and helping children learn to live, play, and work cooperatively. We are also committed to promoting children's self-awareness, competence, self-worth, resiliency, and physical well-being.

the very thought of doping my kid up on some of these unproven,and experimental medications is atrocious! Granted i believe in certain cases medication is the only possible solution....but only after all other outlets have been exhausted! Teachers once demanded my 8 year old niece be medicated with "Vyvanse"  (ADHD medication) before she could attend classes again. Who gives anyone that right? I was flabbergasted! Fact being  she was proven not to have ADHD was what really perturbed me! This is just one of mounting examples of negligence and failure to thoroughly  do one's job. I am in no way invalidating ADHD, for I suffer personally from a very severe case of ADHD...if ya haven't noticed!! ha ha. and that is in addition to Central Auditory Processing Deficiency  (CAPD).which by the way is 86% of the time misdiagnosed as ADHD just a little food for thought. well i have ranted enough!!! i hope I have been of some assistance! I would rather enjoy knowing who this principle is...while maintaining my professional composure,I would like to  assertively ream him force feeding him the plain facts of his commitment,exact nature of his negligence and his failure of the children!!! I pray that your baby boy gets some relief.you must be experiencing unimaginable pain,fear and anxiety  over this situation. the commitment code link I left also goes over his commitment to adhere and respect the child's family beliefs and opinions for increasing the well being of  their child. so study that code a bit, arm yourself with knowledge that is indubitable concerning this issue,and be prepared to take the steps of "going over his head" possible legal action if necessary.Show no fear!!! People that that thrive on it..in a sense you are up against a bully as well...this is a great opportunity to show your son the correct way to deal with a bully!! (not saying you havnt..promise) .....take care Jordan's mom and good luck...thank you allowing me to vent!!! oh be sure to go check out Jodee Blanco's site i left for ya...email her...she is a passionate yet ferocious advocate to have on your side...explain the details to her...she has deep passion for stopping peer abuse.plus she does come and speak at schools..so when you do force the man into submission ( I got faith in ya lady) you'll hopfully have made the connection with her...read her book "Stop Laughing at us" very  helpful. enjoy...