I have been in Education for 20yrs one way or another as a parent and a worker. I have also lived in Dulwich all my life. The system parents have now is far better than what we had 15yrs ago, when one parent could have 5 yes's to a sch and another recieve none. The way the Charter School came about was by a group of parents who constantly fought for a decent local school and never gave up. However, there stills needs to be choice in the local area so that all parents can send their children to the sort of school they believe in. Such as, only girls sch, mixed, only boys and faith schools. My only suggestion to you, is that you gather enough support, have a good enough reason for your need and constantly bagder the council for what might take years. So I agree the time is right when your child is 5/6 and not when they are 11 to start complaining. One other idea that parents may consider is all sending your children to a school that is deemed bad. When schools have children that attend who come from homes that have brought them up to a good standard regardless of income or status that too can turn a bad school around and then everyone wants to go there. It depends if parents are willing to take the risk and go through with it. Good Luck.
