NTA. You are absolutely not the asshole here. First, A overstepped her boundaries BIG TIME. You should (If you haven’t already) take measures to ensure she is not allowed to check your daughter out of school in the future. Second, it seems like you’re teaching your daughter to be responsible and accept the consequences for her actions. M was aware she didn’t have your permission to leave school. Was it tricky that her mother was actually the one signing her out? Absolutely. But! There will be plenty of situations in your daughter’s life where she will be asked to do something she is uncomfortable with/knows she shouldn’t do. You’re helping her learn how to find her voice, whether she realizes it now or not.