What do you do about behavior issues away from you?

As a teacher and a parent, I can tell you that it's the teachers fault. You have offered various incentive systems, you have conferenced with her, you have talked to him. SHE is the one who is not following through on her part. My next conversation with her would be in person with the principal, and it would go something like "I really like you as a teacher, and respect you, but I feel that my FS has issues that are not being addressed properly. I have offered you X, Y, and Z to help deal with them, and you have been very inconsistent. My job is to support you, but I can't do that if he goes to school and what we said we would do hasn't been done. How can we address this going forward, to help FS do as well as possible?" Good luck, and honestly your job is to support what she is doing. If she isn't doing anything, then it's up to her to start something that you can support.