Good for you for standing your ground. I agree, what a joke: she could just up and move out next month, yet in the meantime he's happy to shift the parenting burdens to her. Free nanny!
I can gently point out that unfortunately it's not entirely up to you, but I think you know that. If we wishes to bring her along he will. *However* I strongly feel that the school guidance counsellor or teacher will see things the way you do too. I imagine they would only want actual parents at such a conference. If someone showed up with a girlfriend/nanny, they'll probably be like, "And you are...?"