E.B.
I've worked at a few schools, and often the most troubling times are after school when the parents are chatting and the kids have been cooped up all day and are really wanting to get home.
I'm in no way excusing disrespect or rudeness. But it might help you to observe your son, and maybe understand that its hard for him to either behave or wait patiently while you talk with other adults.
If you know that every day after school you'll be catching up with the other parents, plan ahead. Talk to him about what he can do while you're talking. If he has to wait in the car, could he have a few minutes to unwind with a little game system like a Nintendo DS or whatever portable game systems are popular or affordable? Could he read a comic book before heading home to chores and homework?
I'm just encouraging you to acknowledge that it can be hard for kids who have had to behave in school all day and then have to remain quiet after school while parents visit. Imagine taking several classes yourself, struggling to get to your next class on time, sitting quietly, behaving, dealing with difficult situations, and then just when you get outside, and the day is over, your husband is there waiting to pick you up. But he gets a business phone call and tells you to stand quietly while he discusses a long complicated deal. You'd probably fidget and get impatient. We all would.
So plan ahead. You like your friends, you enjoy catching up with other parents, and your son has been in school all day or otherwise having to be quiet. Talk with him. What could he do while waiting? It doesn't have to be super-productive like homework. It could be a handheld game, light reading, even a small portable container of Legos that you could hand him. And talk with him about letting you know, quietly and politely, that he wants to go. And listen to him respectfully. Look him in the eye, and tell him that you'll go soon. And follow through. Schedule other times when you can visit with your friends. Use the after school time to pick up your son and talk with him. He needs you then, and he's trying to express it (he's expressing it badly, and in an unacceptable manner, but he's trying to tell you that he needs you). Teach him about expressing himself to you in a polite manner ("mommy, I want to tell you about my day." "Mommy, please could I talk with you?"), and then make sure that you listen to him with the same respect that you want from him.