Why are you insisting that the OP is the one at fault here?!
I just can't see it tbh. The OP removed herself and her DS from a DV situation. She has ensured her DS keeps up a relationship with his father and his father is involved with his education. Which would have been Court Ordered had she disagreed, in 99% of cases, anyway.
She does not feel able to spend time with her abuser in this situation. And why the hell should she?!
Yes, I agree that it might be necessary for the OP to explain a bit more than she would like to to the (Head?) teacher, in order for them to understand why a joint appointment isn't appropriate in this situation, but apart from that, I can't see what the OP has done wrong.
You REALLY don't understand Family Courts if you think that violence towards the mother will stop a father from getting unsupervised access to their DC's, PR, and being able to stop them from attending Parent's evenings or being involved with their DC's school.
Unless the violence is so horrific that they have been successfully convicted of it (which is notoriously hard, my friend had two teeth knocked out when her partner hit her face off the metal stairgate, and the CPS felt the evidence wasn't strong enough to continue with), OR the father has been violent towards the child (and often not even then...), it will not be enough to stop PR, unsupervised access, and the possibility of them attending Parents Evenings.
Schools just need to wise up to separated families. I have successfully improved things at my school for those that follow. At one point, before my DS1 started, they refused to even send out two newsletters or two reports. They are now very sensitive to separated families.
DS2's dad has even been given a separate meet the teacher appointment because he couldn't make the first one. It is a 15 min after school appointment. Not long for the teacher, does what DS2's dad needs.
I really think that certain teachers need to stop victim blaming and living in the 50's...