6 year old missing school

My son’s mother just informed me she is now planning on traveling with our son for her half of Thanksgiving holiday visitation. Our son is with me for the first half and goes to his mother on Friday evening. She said they will leave either that evening or on Saturday morning and driving back the following Monday. My son is supposed to be in school on Monday as the Thanksgiving break is over on Friday so it is not an extended holiday weekend through Monday (school is out Wednesday through Sunday, school resumes Monday). I told her I do not agree with him missing school and that obviously I love that he is getting to visit with family that he rarely sees, but I do not want him missing school. We have split legal custody by court order and are ordered to agree on all major decisions with education be specifically included in that.
When our son was in preschool which I paid a lot of money for, she did this same thing a couple years ago, but he missed an entire week of school that week.
She has said that there is nothing I can do about it and it’s “not a big deal” for him to miss a day despite the fact that will put him behind and his teacher agrees he needs to be present unless health issues arise. Is there anything I can do to prevent her keeping him away when I don’t agree with it and with him missing school? If she does it, is there anything I can do afterwards, to prevent this from happening in the future?

Unfortunately there is no legal remedy that will be able to address the issue by Thanksgiving. If she breaches the order, you can file a motion for order to show cause and seek to have her held in contempt for not abiding by visitation terms laid out in your court order. If you anticipate this being an issue going forward, you could also file a motion to modify custody based on breach. As an aside, your next custody order should address attendance at school so there is no confusion about when the child should be missing school.

Thanks so much!
Follow-up question: Since my last post, I have spoken with my son’s mother on the phone. She stated it’s just kindergarten so it doesn’t even matter anyway. I suggested that our son’s visit with me be cut short a day early and he could return to her on Friday morning if she would be willing to leave town a day early so they can return Sunday vs coming back on Monday like she is planning. She said ‘He would have to come back by 8.’ I said that was perfectly fine. I assume she did not expect me to agree to that because her response was ‘I just don’t want to do that.’ She got very mad at this point and I ended the conversation because I could see it wasn’t going anywhere. I have this entire phone call recorded.
Do I need to email her to go back over my proposal for him to come back to her early in order to prevent him from missing school so there is email proof of me providing this to her? Or is the phone call sufficient documentation if we end up in court for me to prove I gave her an option for him to not miss school?

The phone call would be sufficient. North Carolina is a one-party consent state so you can in fact record conversations so long as you are involved in the conversation (essentially, you can consent to the recording). You can always email her to reiterate your position, but so long as you can lay the appropriate foundation, the recorded call would be admitted as evidence.