I think it is time to escalate to the Council's complaints and write to your local Councillor. We had an issue in primary school with visitors and while lots of us complained to the head nothing really was done just vague platitudes. The parents who went straight to the Council were offered meetings, invited to sit on future groups about diversity, and were taken very very seriously. So I know what to do now, ask the school for a response, if it is ineffectual immediately escalate.