Help with my DS!

[mlrw70]

Okay DS is 5 and he is a great, sweet, and amazing little guy. I love him so much. But seriously, he is getting in trouble at school EVERY day! He can't/won't just stop talking. He talks back all the time! Most of the time it isn't in a snotty way, just a "have to have the last word" way. I've been working with his teacher and we are doing a short-term and long-term reward thing. He gets 5 pom-poms to keep during the day and when he talks back he loses one. If he can keep at least one he gets a check mark on a chart at home to earn a "big" prize - (Chuck E. Cheese).
But it doesn't seem to be working, and I am at a loss. Any suggestions ladies