Having been a governor of two schools which were required to turn around a poor ofsted report (I was an additional governor appointed because of my experience) I can say that the huge amount of issues listed by ofsted at HH is not a quick fix. It might look easy to a parent, but believe me, this is climbing Mount Everest in terms of turning a school around.
If it was me, I would consider selling. Or bringing in experienced senior leadership. Where such people will come from is a other issue. There would be a financial cost in doing this and an increase in fees would, I think, be necessary. There is also the problem that this is not just about writing policies or sending the staff off on training courses. You have to embed a new way of doing things. No-one appears capable of introducing a performance management policy let alone carrying it out. No-one checks the quality of teaching. This begs the questions, does anyone know what excellent teaching looks like? How will this lack of knowledge be tackled? It is absolutely fundamental to the success of the school. How will the school insist that children's work is marked in a constructive fashion to enhance learning? In a school this size, this will be a huge job. Although the parents do need to get a grip, a lot of issues in the report are technical issues for teaching professionals because parents cannot introduce performance management or embed a method of progress checking that informs lesson planning, for example.
I do advise parents, however, to try and find out how good schools actually do this so you have a view of what the revised HH should look like. The school should now produce an action plan, so I would expect to see every issue under close scrutiny with a detailed plan as to how improvement will take place, costs, when it will be completed, who is responsible and what success will look like. Are the current owners capable of doing this?
ReallyTired is absolutely correct that when parents only see one school, they are rarely well placed to judge the education on offer there and they also support the institution that they chose and have spent a lot of money on.
That is why failure on this scale is so devastating.