Maybe you could tell your daughter that you need to double check with your ex and the school before thinking about letting her walk alone because you do not think that is allowed at her age. You could ask ex and school to support the message that your daughter is too young to be out on the streets by herself. Once it is clear that all are on the same page, you would be in a position to exercise parental authority when you pick her up. Tell her that you have to walk together. Your ex has created circumstances where your role as dad is compromised, you have to tread so carefully. But, I think this is a point where you can insist. It would be good to find careful ways of showing your daughter some decisions are not her's to make.