Can anyone help with my son?

He is nine years old, already "knows" everything and he is suffering in school. I instill discipline, as well as participate in many activities. All to no avail. I don't think he is "dumb" when it comes to school, but he is not learning up to his full potential. He sort of acts like we did when we were well our teens. So I am not saying I was perfect, but at nine years old? Any answers would be greatly appreciated.