It's anecdotal, and I can see other people have suggested similar. DD is year 11 now and very strong academically. Her written English took a massive dip over Covid - from reports saying achievement was extremely high in Year 4 (or was Covid Year 5?), to actually failing a Year 7 entrance exam because English was so bad by middle of Year 6. Reading and oral English were not impacted, it was purely the ability to communicate in writing that nosedived. I don't think it's coincidental that it's the one subject that we just didn't know where to start on teaching it ourselves. She does have ADHD, which probably made it harder.
We've got her there now, and she's predicted 9s for GCSE but this is because we brought in a tutor to specifically focus on how to express thoughts in writing as soon as we realized (so towards the end of year 6) and it took until the end of year 9 for her to be getting reports saying she was predicted above a 6. If she hadn't sat that specific entrance exam where they actually required her to put pen to paper and write something, we wouldn't have realized it was as bad as it was, because we simply didn't know what level to expect. She passed other entrance exams easily because they were CAT4 style so required high level of understanding of English but didn't actually require writing.
The secondary school she goes to isn't very academic but the 7 Head of English insisted that there was no need to tutor and her profile was normal and not reflective of an issue, and her actual English teacher just told us that she needed to 'read more' (she had maxed out on reading age tests at the time - reading ability was not the issue) and we had a literal Ed Psych report to show what her actual profile was.
That might not be what's happening here, but this is a cohort who would have had Covid impact them during year 6 and year 7. If it's a cohort with strong reading ability, and a not great English department, it's completely possible that they realized too late that there was a real need to focus on written English. Often highly academic schools just don't know what to do with kids who won't get at least a 6 without any effort. Younger kids, who had the Covid impact right at the beginning of primary school, and who come from socioeconomic groups where there is a highly supportive and academically focused home environment, won't have been impacted in the same way. We wouldn't have had an issue if what was needed was phonics, Jack and Jill books, spelling and letter formation.
Or it could just be second language English speakers, but I think that it's likely to be more than that. I'd also suggest that the parent group may very single mindedly focused on STEM and see English as a backburner subject as long as the kid hits a 7, meaning they realized too late and didn't bring on the English tutor in the way they would have brought on tutors for science or maths in a second in the same situation.