Not only that but the OP refers to the Department for Education data on Forest School from 2019 (the last year of public examinations, before they resumed this year). It's true that its progress score for A Levels is "below average" at -0.29, as the OP is at pains to point out, but beyond that headline is the DfE small print which explains those progress / value added. Crucially, this DfE small print states:
"These figures tell you how much progress students who studied A levels at this school or college made between the end of key stage 4 and the end of their A level studies, compared to similar students across England... A score below zero means students made less progress, on average, than students across England who got similar results at the end of key stage 4.
A negative progress score does not mean students made no progress, or the school or college has failed, rather it means students in this school or college made less progress than other students across England with similar results at the end of key stage 4. The majority of schools and colleges have progress scores between -2 and +2. These scores are also known as 'value added' scores."
You can also see from that DfE data that the average A Level grade at Forest School in 2019 was a B: a B at A Level is hardly "truly dreadful". It's not excellent, but as stated upthread, Forest has never been one of your top flight independent schools where the average is more likely to be A* or A.
The OP also made this point upthread: "Hopefully, the School will have very significantly improved their dreadful academic score and of course I'll be the first to applaud. If they haven't, I hope my detractors will be similarly magnanimous".
Well, one doesn't have to wait for DfE data because as I've indicated above, the school has already published its A Level pass rates last Thursday on their website, which indicates an increase from 39% A (star) and A grades in 2019 to 67% A (star) and A grades in 2022 - an increase of 28%. For A (star) to B grades, they've increased from 65% in 2019 to 85% in 2022 - an increase of 20%. And these 2022 figures are either higher than or level with the internally awarded A Level grades for 2020 and 2021.
Whether or not the increase in 2022's pass rates merit the DfE's calculations lifting Forest from "below average" in its progress / value added score in 2019 remains to be seen, but I would say on the basis of this data published on Thursday alone that Forest has "significantly improved" on its results. Unless the OP is doubting the veracity of the school's data published on its website, of course.
And to repeat a point I made upthread: I don't have any particular interest or attachment to Forest School. In fact, I barely knew of it until I started reading some of the OP's posts on this forum. What I do have an interest in is when people post information very selectively in order peddle a particular agenda about a particular school.