I appears to me she has some valid points. Your son is very lucky to have you and your husband and I am sure with your help he will catch up. However, I think he needs to hear from you that althought is is great he is having a great time making movies and he can continue to do so in his spare time, it is important to also do the assigned work at school.
I may be alone on this opinion, but I do think that kids need to learn to do something the way they are asked just because. Following directions even if it seems ridiculous at times and even if you don't feel like it is still an important skill. It amazes me how many kids have a hard time with this (including my own DS4). Anyhow, I am not suggesting busy work and that kids should stay in classes where they aren't challenged, but I think that sometimes gifted kids have a harder time with just doing things that an authority figure told them to do. I theorize that part of this is because we praise them with their "oustide of the box" thinking when sometimes they are supposed to be in a box. Wow that was a tangent...but just a thought.
So back on topic...maybe if you could ask her if there is anything in particular that she could assign him and he could do at home (that could be used towards a grade). That may work.
