I would say that keeping a good grade implies that one has already earned it. Kids should understand that disrupting class results in lower grades, but they should also know exactly what they have to do to earn those grades in the first place. If they walk in the door with 100% already, then all they have to do is either a) be absent from class, so that they don't do anything to screw up their grade, or b) sit there and try not to cause a scene. That isn't really inspiring them to DO anything.
Think about our lives as adults. We don't get paid to just show up and not cause trouble. We earn a salary in exchange for our performance. Imagine a boss saying, "Okay, I'll pay you this salary if you agree to come into work and not disrupt anyone else. Any mistake you make will result in lowering your salary." I'd imagine most folks would fall into two camps - some (many) would accept the situation and be fine with it, since it's easy money. But others would feel insulted, and probably unmotivated to actually do anything, and even bored enough to start thinking of pranks and jokes they could play at work to make life more interesting. Especially if they hated work and didn't want to be there to begin with. Have you ever met someone who WANTED to be fired? That's the situation a lot of our kids are in. They don't particularly want to be in our classes, or in school in general, so the negative consequences don't particularly deter them, especially if there is no incentive to actually try and do well.
A point system is extrinsically motivated reinforcement. You do the right thing because the system gives you some benefit you really want - a good grade, a salary, more positive recognition, whatever. By the same token, to avoid a problem, you'll do things that aren't particularly pleasant, like paying the bills or driving within a reasonable distance of the speed limit, to avoid fines and other unpleasant consequences.
It seems like your system is counting on the kids to be intrinsically motivated - that is, to want to do the right thing, and to have the respect and foresight to avoid the consequences of doing the wrong thing. Yes, middle schoolers would hopefully have this in place - but some don't. That's why you're taking points away in the first place, isn't it? In an effort to communicate that their behavior is unacceptable, and give them a reason to avoid the misbehavior? Taking the points away clearly isn't enough of a deterrent. In fact, they seem to have decided that it's more rewarding to have the points taken away, but have immediate fun and social attention in class.
What would actually motivate these kids to want to behave in class? What can you, as a teacher, give them that their friends can't? You have power over their grades, but they seem to be in a power struggle with you, and are telling you through their actions that it is more important for them to save face and not submit to authority than it is to get the 100% that they haven't really done anything to even earn yet. Maybe they are pretending not to care about their grades because they don't want to be embarrassed in front of their peers. Or maybe they really don't care. So, fine - if they don't care about grades, what do they care about?
This is a tough situation, and this age group isn't easy even if the kids are basically motivated to do well. But if you can crack open even one kid's thinking and figure out how to break the spiral, I bet the situation would improve a lot. Regardless, though, for your more motivated students, I would at least offer an extra credit option - a way to recognize them for their hard work.
