The OP's mental health aside, I totally understand how upsetting this is, when you are very keen for your children to get a good education. You rely on their school to do a good job...and when you read that their effectiveness, teaching and outcomes for children require improvement, your heart sinks.....because you want the school to be Good or better and you want your child to be being exposed to a school which is good in all areas, not one where there is high teacher turnover, or the staff don't consistently follow their own policies, or where when they are graded poorly, the school tells the students it was a great report, or when contacting the parents isn't prepared to say there are significant things that need work, rather than making excuses.
I understand why you're upset. I would be too. And yes, your individual child's experience to date is very important - if they have been happy and seeming to make good progress, then that's really good and yes, Ofsted doesn't count for everything at all, but a poor report just makes you lose at least some confidence in that school, to deliver the best possible outcomes for your child.
I wouldn't have a knee jerk reaction, but perhaps ask for an appointment with the Head to talk through their response to the report....see if they are defensive and in denial and if they seem to have a good plan for moving forward. And then decide if you might want to look at alternatives....it's difficult because offend there aren't actually many alternatives, but usually somewhere can be found,me specially if you're prepared to facilitate travel.
It may well be that Op has had serious problems connected to her issues with the school before. She must guard against becoming over-invested in this issue, but at the same time, it's fine to feel really disappointed and let down and to want to investigate further.
And I think I might know the school being spoken of....in an affluent area, although like all large secondaries, with a range of children....but should be doing better than it is.
And I feel for the staff at that school. Because this will be so upsetting for them and mean all kinds of changes, which are clearly needed, but will be unpleasant in implementation.