My bright & challenging daughter with ADHD started middle school this year and oh boy, adding 12 year old behavior on top of all the other behavior is more than I can handle. I could really use some support from moms who've been there. :-)
I am not a mom of middle schoolers (my boys are 3 and almost 5) However, I taught middle school special education classes for awhile so I can relate some to that age. It is a difficult age. They are kind of stuck somewhere between adulthood and childhood and don't know who they are. They find this attitude that is over the top and very sassy. (at least the girls in my class were). I think just structure and clear expectations and consequences are your best bet. Just be sure to follow thru always or else you will just be creating arguments later.