I teach at an all girls high school and there was just a huge level of disrespect because they love to talk and use their cellphone so this is how I handled it.
For their English grade, they have a 10% participation mark which has been given to me so at the end of every class, I check to see if they've completed the assignment. They get a check mark if they do the work and an X if they don't. That pretty much made all the girls work but some were still being lazy so I upped the ante. The teachers have these yellow cards they give the students if they're bad. On the yellow card, there are different things they've done - i.e. talked back to the teacher, used their cell phones and beside it there are numbers. Basically the numbers are the points the students will lose towards their final grade, it's quite serious. So, if a student has 3 x's in my class, they receive one yellow card. If they maintain a perfect score, they'll get a green card (which, of course, is the opposite of the yellow card)
If a student receives a yellow card, it goes straight to their homeroom teacher and I have a talk with the homeroom teacher, then they speak to the student. Students seem to be very scared of the homeroom teacher. I've never had it gone beyond that, but if it does, the next step would be to send a text (Yes, a text. My school sends text messages because they believe it's better than sending home a letter) to their parents.
For me, I tried the whole reward system and it just didn't work. For my students, there had to be a real loss in their grades for them to straighten up.
I know you're in a middle school, but grades are still important, especially when they have to show their parents. I suggest talking to your co-teachers to see if there if any way you can get some of those marks or at the very least, have them use your lessons in the exams.
Sometimes these kids just get so out of control - and you shouldn't have to spend your money on rewarding these kids for behavior they should already have instilled in them.
My whole system may sound super strict, but it's the only way to get the students to do the work. Otherwise, they see your class as pointless and continue to do whatever they want.