9 year constantly getting into trouble for misbehaving.?

My 9 year old son is constantly getting into trouble at school. He is constantly talking and just doing little things in cl. He is an A & B student and is very intelligent, but he constantly talks. His teachers do a check mark system, whereas, everytime he gets 5 checks in one day, he gets a detention. I thought it was because maybe he is bored in cl, but because he has had so many detentions, he is not eligible for gifted cles or the gifted school. My question is can anyone give me some good advice on how to get him to stop the talking and getting into trouble. He has gone so far as to forge my name on his detention slip because he did not want me to find out. I am at my witts end with him. I have tried him and taking away his favorite things. My 6 year sees his brother getting into trouble so much, he is afraid to. By the way, I am divorced and my ex-husband does not have a very strong role in his life. Can you help me out??? PLEASE BECAUSE I AM LOSING IT.