A bully and some books

I am a member at the YMCA and use there facilities mainly to exercise and other times to swim with Jada and the family in the pool. I decided to join the Y because of the babysitting included in the membership cost, so I can exercise at my convenience and not have to worry about my exercise routine being dependent upon my husband's availability.
Within the past couple weeks, when I have picked Gabe up after exercising, the teacher's have mentioned that Gabe is taking other children's toys and not following the rules (i.e. climbing up the slide, instead of going down.) At first, I found it rather annoying, because he is a 21 month old boy and they don't know how to share and they need to be reminded often about rules of proper use of toys etc. Then I was a little concerned after a visit when they mentioned that my son laid on top of another child to get a toy they had, and how he was laughing because he thought his behaviour was funny, not naughty, when the teacher got after him. And finally, this morning, I was interrupted during my workout because Gabe was taking toys from kids, and when the teacher removed him from the situation and used a "stern" voice to tell him not to take the other child's toy, he took a swing at her. Then on top of that, he through himself down and had a tantrum, only they were concerned because he kept trying to bang his head on the floor.
I am a little flabbergasted and don't know quite how to correct the situation because normally if he starts to behave like that I am right there and intercede before anything really happens of that nature, but he has been more prone to tantrums etc, but I know that is just a part of his developmental stage at this age. Anyone have any advice or suggestions for me?
I finished reading the Golden Compass by Phillip Pullman. I started on the second novel, The Subtle Knife but haven't finished it. I thought The Golden Compass was a good book. It has strong themes that teach against the idea of religious authority etc, which makes sense, because if I remember correctly Mr. Pullman wrote these novels as his atheistic/agnostic version of C.S. Lewis' books The Chronicles of Narnia. The funny thing is, is that from reading Mr. Pullman's first book, even though it is strongly felt that he has no respect for religious organizations, the book strongly suggests a Higher Power of some sort, though by reading just a few chapters of Mr. Pullman's second book in the trilogy, it appears that one of the character's has taken it upon himself to find God and kill him. I find that thought very disturbing, and haven't been able to pick up the second book for a few days. Don't know if I'll finish it. I haven't decided yet.
I've been reading a lot of children's books lately too, as Jada's homework assignments is to get in 10 mins of reading everyday, whether it is me reading to her or vice versa. We read a book called "A mama for Owen". It was very sweet and is a true story. It takes about 5 minutes to read, and is about a hippo from Africa. Go to your local library and check it out when you get a chance.
And one last thing, I just wanna say how expensive it is to have to pay the dentist when you have no insurance! Yikes, I think I'll have to become the dentist's indentured servant for 5 years just to pay off the bill.