I am dealing with one biter/aggressor and it is wearing. I can't imagine having more than that in a group.
I agree with the advice you've been given. Can you pinpoint who is the alpha? And who is trying to be alpha? I try to keep those personalities separate at all times to reduce the conflict.
I would also chat with parents, maybe send home a little note, about simple techniques to do at home to help them in care. We do a lot of simple phrases: my turn, all done, help please (for when their words aren't working), etc. It takes a lot of work but I am starting to see a bit of progress. I need to stay right in the thick of things and can't turn my attention away for long. I try to create separate play areas so that I can separate personalities and give them a chance to regroup before rejoining friends.
In my case, I am able to term if I don't see improvement (and just did), because it is completely stressful and wearing. I would continue to speak to your director about your concerns. That's not a fun environment for anyone.