@FallingAutumnLeaf asked: How has it gone from being set 4/5 to the school having 9 classes at GCSE?
I think OP was looking at a previous year's results, and it may have been a year that was larger than her daughter's year. Where I live the year that has just taken GCSEs was a bulge year, and so was the year 2 years above - at secondary school DS1's year was increased to 9 forms from 8, and then at the start of year 8, to 10. And the school isn't over or fully subscribed. DS2 is 2 years below, so now in year 10, and I think his year is back at the normal 8 form entry.
The school doesn't use sets as such even for Maths, though I think at GCSE, the choice of options, the subjects taught in tiers and the choice of Triple or Combined Science does effectively divvy students up. Combined Science is also taught at Higher and Foundation tiers and DS2 has chosen this, saying he thought Triple would be too much work but I think actually it gave him 4 options rather than 3. DS1 had two rounds of Mocks in year 11 to confirm being entered at the right level, and for all subjects so students/teachers got a good idea of where intervention was needed.
I know quite a few parents in my area were put off by the lack of sets at school but it's not the only way of differentiating/encouraging students to do their best, and like grammar schools, it hopefully doesn't split off everyone who doesn't benefit from being in the most successful group at the start. I'm in my 50s and started secondary school many years ago, and what middle school we'd been to (so mostly, whether or not we lived in posher catchment areas) mattered more than it should when it came to placing us in sets, and this is certainly an issue where I live now, mostly working class with some gentrification (but very uneven), very diverse, etc.