My first teaching job was eighth grade English, and I experienced exactly what you've described.
Eighth graders are at a peak in terms of questioning authority and wanting to impress their peers.
They are at a low point in terms of inner motivation. I truly admire the teachers in this discussion who have found successful strategies for motivating and managing eighth- graders. But I do think you should seriously consider switching to a lower grade level, at least for awhile. I bailed out of eighth grade, got recertified in elementary ed, and never ever ever had another teaching day with the kind of frustrations I had experienced with the eighth-graders. But I also realized that I would be a better teacher if I could focus on one main group of students each day.