I'm betting it's a mistake. My dd had zeros in a few assignments and they were teacher error.
My DS who has dysgraphia had a jerk for a teacher who gave him zeros at times to motivate him, but that's the exception IMO.
What does "z" mean? Does it maybe mean turned in but not graded yet? I'd wait until she gets it back. And not helicopter. Your dd will handle it. So what if it is a zero and she cries and is embarrassed? She can learn to talk to the teacher and hash these things out. My kids do this on their own. I intervene only if they have reached their end point. They have my support and advice and they feel very good to do this themselves. Plus,the teachers appreciate it. (Of course, your school may be totally different, and your daughter may be totally different!) It's middle school where there is freedom to learn these things.
This reminds me of an assignment my ds had recently. They were to create a 3D model of an animal cell and label each of the parts. DS worked on it for hours over a period of a day or two, and did a beautiful job. He was finished early (unlike my older ds who was always energized by waiting until the last minute). The teacher had the class first grade their own work and then each project was to be graded by a peer. Well, ds thought it would be "braggy" to give his project a perfect score so took a few random points off. And the peer that graded his project after followed suit. Gave him the same points off for the same "issues". The teacher recorded the grade. DS was upset. He felt he deserved a better grade.
Now regardless of how I feel about peers grading work in 5th grade, the biggest lesson for ds was to value his effort and work and that false modesty is not only unnecessary but also can bite you later. I would not go in and talk to the teacher about it, and I feel badly about it in some ways, but also knew that the lesson was more important than the grade.
*However, this is different than the OP's situation.
