Violent physical harassment of a socially awkward kid

A boy in my kid's school is being violently beaten (punches and kicks) by a bunch of otherwise normal kids. There is always a different crowd instigating the beatings, but there are usually more than five of them. Here are few facts/observations:

• the bullied kid is never fighting back, rather being knocked down and beaten
• he is from a 'special' class where they keep mostly autistic kids or those who have learning difficulties
• he has a reputation of being weird but very nice and harmless
• lots of peers who do not participate in the beatings find the while thing very disturbing
• lots of kids observed that the teachers are aware of the situation but are indifferent and don't do anything.

Now, I brought it up with the teachers as soon as I've learned about it and they expressed their frustration with it. While acknowledging the occurrence, they put the blame on the participating kids and the principal. They also claim that the bullied kid is a bit weird and asks other kids to beat him up. Nonsense. In the mean time they encouraged me to disk with the principal. I have a meeting with the principal arranged, but I'm perplexed. Any thoughts?

Of course I'll state the obvious to the principal and demand action and follow up - but what are the key points I might be missing?