How to handle a wild man

Okay, help!

I feel like I work so hard at this trying-to-be-a-good-parent thing. I encourage my child to eat a variety of healthy foods, including fruits and vegetables. I expose him to culture by taking him to museums, botanical gardens and even a play last weekend. I read multiple books to him every day. I take him to Sunday School every week. I never used baby talk when I talked to him. I praise him for the things that he does well. I set boundaries when it comes to behaviors that are not acceptable. I make sure he gets plenty of sleep. I talk to him. I laugh with him and tell him I love him all the time.

And I try, I really do, to be patient with William when he does stuff that drives me crazy. So, why, oh why, does he insist on doing things that drive me nuts? Am I not responding correctly?

Here's an example. When I walked into his classroom to pick him up from school today, William acted like a hurricane. A fast-moving, destroy-everything-in-his-path hurricane. He began screeching happily and tearing madly around the classroom, knocking toys away from a couple of his classmates. He pushed one little boy who was trying to use the potty. He tore around the room and refused to stay still long enough for me to wrestle him into his coat. It was maddening. And exhausting.

And it's like this nearly every single time I pick him up from school or the nursery at church. None of the other kids act like that, and I have to admit that I get a little embarrassed. I worry that the other parents are looking at me and thinking, "Why doesn't she control her child?" I worry that no one is ever going to want to play with my son because he's so wild. I worry that he's not going to have any friends because the other kids are going to be afraid of him! He's not mean, but he gets so rambunctious. Why does this happen? What can I do about it?

And the other thing is...he can behave so beautifully when he wants to. On Saturday, I took him to see the play "The Wizard of Oz" at the Belcourt Theatre down in Hillsboro Village. He was an angel. He sat nicely in his seat for the first part, then in my lap for the rest of it. He didn't yell or scream or fidget or get down and run around. He used his "inside voice" when he wanted to ask me a question. He behaved better than most of the older children who were there. And yesterday at Cheekwood, he behaved very nicely inside the museum. He didn't run off or screech or keep doing anything that I asked him not to do.

So why does he does act like such a madman at church and school at pickup time? And why don't any of the other kids? What am I doing wrong?