Ah, Lord help you - welcome to puberty.
It sucks.
My son went through this. I just continued to repeat the expectations, enforce the consequences, rallied his teachers and our family, set good examples and plugged away at it.
You are not to feel guilty or stupid. I think every pubescent boy goes through this to some extent. Our children are going to be exposed to a myriad of things - if the core values are in place, and consistently, reinforced, you are halfway there.
He will rebel, he will balk, he will be cruddy and rude. But, don't give up and don't blame yourself. Just keep being the Mom and keep those lines of communication wide open.
Engage him other activities - esteem building, team building, activities. Continue to show him the better path, lead by example, and explain that not all behaviors are good behaviors - no matter who is exhibiting them.
The one thing I finally did with my son, in 6th grade, was explain to him that his grades were on him. I assured him that I would give him all the tools and support he needed to pass, but that ultimately, if he decided to not do the work, I would also ensure that he repeated the 6th grade. No summer school, no excuses, no begging the school to pass him on. He would simply repeat. Since that idea was appalling to him, he managed to pull everything up and passed. It was a hard lesson for me, to let him "go", but for him, a strong one. He is no a junior and understands that I will not provide him a safety net, or excuses for his failure. He does much better in school and "owns" his results.
I learned that I have to sometimes let him fail on his own in order for him to understand the consequences. Normally, we do this on the little things - it is much harder to consider it on the big things. But, for my stubborn son, it was the magic key.