Unfortunately these days, everyone is afraid of getting sued over the littlest things.
Whilst I agree that a violent response may be in order, I also advocate it only as a last resort.
The school administrators have had ample opportunity to remedy the issue, but have seemingly failed to do so. I assume you have copies of all communication with regard to these incidents, as well as documented times and dates of phone calls over these events.
Personally, my approach would be twofold.
First, on my meeting with school officials, I would provide copies of all information I have in one neat, clear, and concise package. I would inform them politely but very firmly, that their inaction is causing you and your child distress, and negatively affecting their learning environment. Hint at legal action if no action is taken to remedy the situation.
Give precise timelines for your requests to be implemented, and follow through completely.
I would also prepare your child to physically defend themself should the need arise.
Personally, through most of my school days I was the bully target, simply because I wasalways the biggest kid in school. My dilemma being that if I beat someone down, I was the bad guy picking on the smaller kid; or, if I let them whip me, I was a wimp. No win situation.
It finally ended when three guys decided to pick a fight with me at recess during a pickup football game. I ended up with a split lip and black eye, but they all fared much worse, with one losing a tooth, one with bruised ribs, and the third with a concussion.
That was the last time anyone saw fit to pick on me.
Self defense is a natural right, regardless of what any reputed "authority" may have to say.
You have essentially two options here. You can stand up and take decisive action, or roll over and give up. You don't seem like the roll over type to me.
Cover your bases, promise them action, and make it happen with extreme prejudice.
I RIDE, THEREFORE I AM
COWBOY UP OR GO CRY IN THE TRUCK