How to deal with a child who is lying?

I have a 7 year old son, he has been known to lie off and on. If it's going to get him in trouble, he will lie to try to get out of it. Well today I got a call from his teacher, he is misbehaving at school. I guess he threw a pencil across the room, almost hit another child and told her he didn't do it. She said she had seen him do it. I have told him lying is unacceptable. It is always best to tell the truth, even if it does get in trouble because it will only make it worse if you lie. I told him nobody likes a liar, and no one trusts a liar. I am a very honest person who will admit when I do something wrong, so I think I'm a pretty good role model. How should I handle this? I know I have to have a talk with him after school but I don't know what more I can say or do. He was supposed to have baseball practice tonight, we won't be going to that now. I don't want him lying and I especially don't want him getting in any more trouble at school. I found out 2 months ago I am pregnant, I'm thinking that might be why he's acting out? He is an only child and this could be causing more stress on him. Any suggestions please help!!