Well, I went through this too and I found an unusual solution. I recommend that you read the book, Raising Our Children, Raising Ourselves by Dr. Aldort. She talks about it in the book and her solution is unlike anything I have ever seen and it is such a relief. You should read it yourself, as when I say this, I generally get negative reaction because everyone is so used to the old ways, and toddler groups... so please don't jumt to a conclusion when I give you a summary:
Aldort points out to research that shows that toddler who do not engage in peer grouops do not tend to develp such behaviors. She says that putting toddlers together is like putting the blind with the blind. They get frustrated and they learn bad social habits and they learn to depend on adults too much for their relationships.
Well, my oldest was a toddler in a toddler group when I encountered Aldort's ideas. I stopped going to that group. I started socializing with moms when my husband could stay with my son. It was such a giant relief. Oh my. Unbelievable. No struggle, no embarassment, and over time, all the aggressive behaviors vanished. Mixing mom time with toddler time does not work, and toddlers with each other does not work either. My son does best with older kids, one on one. Try it. Read that book. You may be surprised.