11 yo daughter's school work going down hill

My daughter (11, 6th grade) has been getting progressively worse on her grades the last month or so. She's been getting 50-60 on tests she used to get 80-90 on. Most of the issues on math were sloppy mistakes, but she just got back a social studies test and got 10/20.

What's the best way to proceed? We have already told her if she does well she is rewarded and we took away electronics when she got a 60 earlier in the week on a math test. Today was the news of the 50% on her social studies test. I don't think she is studying enough, so that's a start, but is it too soon to think about scheduling a teacher conference after a few weeks of these poor grades? Also, to what degree should negative punishment be used? Would too harsh a penalty for the second failing grade in one week push her to be even more apathetic?

Suggestions would be appreciated for anyone who's gone through this. She's definitely capable of much better, but she's also very stubborn and defensive.