I'm mad for you!
Do you have a student handbook handy? If so, what policy does the school have for bullying/violence?
Can your DH go with you to the meeting? It would be great if he could to provide support for you. It's going to be a tough meeting. They have a history of minimizing the problem. Treating you as if you are the problem. You have to turn it around. Do your homework on the school policy & goals, school districts policy & goals. Write them in your notebook.
I bet if you sat down with your DS you could get the dates of the other incidents between the two of you. It might be of importance later.
I'm from C.Ann's school of thought. Get everyone's name and title. Write down everything stated during the meeting.
Tell them your DS doesn't want to attend school anymore because he doesn't want to deal with the bullies. Tell them you are upset, because it's their job to deal with them. And they have failed him. Ask them if alienating their students is one of their goals, because they've succeeded.
Let them know what has been occuring is not acceptable. Ask what they plan to do in the future to insure these incidences stop. Come prepared with what you want to occur to insure your DS's safety, just in case they don't have an adequate plan. (highly likely they won't have a good plan or follow through)
Know your next step if you are dissatisfied with this meeting.
This is what I would do. Good luck and keep us posted!