My response to this is based solely on experience, and not a legal opinion what so ever. I am not attempting to hi-jack this thread, but provide you with my experience for future prevention.
I recently went through a relatively similar situation as you. My eldest child starting missing more and more school each year after i moved out of the house. The more that i complained about it, the more that everyone found ways around it.
First, the ex and my eldest went to court and asked that she not have to spend nights with me anymore when school was in session (albeit none of her absences were when she was with me) the court allowed this.
The following year her absences doubled for the year. I dealt with the school trying to find out why i was not being notified of these absences so that I could be more proactive. They could not answer my questions, instead they told me that since her grades were still good, there was nothing that they could do.
Finally, this year, I attempted to get a handle on the situation as early as possible. in the first month of school she was up to 7 absences. she had 14 absences at the end of the first marking period. I called Child protective services hoping they could intervene, they told me the school had to start the process. I contacted the school and was told that since "mom" was writing excuses for her, they could not do anything because legally the absences were excused. the attendance policies only address un excused absences. I finally convinced the superintendant of schools to have a meeting to enable all parties to meet and find a way to get the child to school.
At this meeting, my ex allowed my daughter to drop out of school. At our next court appearance I brought the matter to the judge at pre-trial and the law guardian lied to the judge and said i was on board with it and that my relationship was better with daughter as a result. This couldnt be farther from the truth. but because the law guardian said this, the judge didnt care that she dropped out. And the court didnt care about her attendance issues what so ever.
With this being said, I apologize for being long winded, and if there is any legal avenue that might be available to you, I suggest that you do not hesitate at all, or else your daughter may eventually drop out of high school as well and there will be nothing that you can do about it.
Again, I am not attempting to hijack your thread with my story, but hopefully you can learn from my situation and prevent it.
Good luck, I sincerely hope that this helps.