I do think there is a lot of naivety on this thread- the idea that all children get qualified teachers at GCSE and A-level is definitely not true!
Yes, to some extent, schools will protect exam years wherever they can- but this really means Y11 first, maybe Y13 second, with Y10 a distant third. And sometimes, especially if a teacher leaves mid year, it's not possible to protect even Y11 classes. A few years ago, I taught a GCSE science class which was a shared group between two teachers (in theory). However, there was no-one to teach the other half of the lessons, so they went through about 5 different teachers (plus multiple cover/supply lessons each term) in that year. It definitely had a huge negative impact on their GCSEs. That same year, two Y10 classes were in the same situation as well.
In order to try and help, my timetable changed multiple times that year- that meant my KS3 groups and my Y12 class had changes too, and in some cases multiple teachers and periods of supply throughout the year.
Many of those students definitely got far lower grades than they otherwise might have done!
I know schools who now make timetable decisions based around how many lessons they can staff, not what's best for each subject. I know schools where class sizes for e.g. triple science or A-level are massive because that's the only way to get all the students to have subject specialists.
For those saying that the lessons or scheme of work etc are provided by a qualified subject specialist- yes, that may be true, but that doesn't help if students have challenges/questions/misconceptions in the classroom AND it also increases the workload of that subject specialist.
If I'm teaching your child in Y11 who didn't have a science teacher last year, you probably want me to have time to put on revision sessions, to mark additional pieces of work, to really plan what I'm doing and tailor it to the gaps that class have... I can't do that if I'm producing KS3 resources for a non-specialist to use at the same time. If I'm teaching your child in Y10, and they don't have a specialist for all their lessons, then all the marking etc for that class falls to me. If I've spent all weekend marking more than "my share" of mocks, then the lesson I plan for the non-specialist to teach is probably not going to be my best work...
None of that is made up, by the way, it's a real situation I've been in.
BTW, they would have got qualified subject specialists at A-level, but a lot of the students in that Y11 class who wanted to do science A-levels ended up not being able to get the grades they needed to study them....
I've now left that school, moved to a school where it was promised I would only teach within specialism, but ended up this year teaching Y12 a niche A-level subject which I'm not qualified in at all. But the qualified teacher of the subject can't cover it, and the other option is an A-level class of 35. I know that I'm doing those students a disservice in lots of ways, but there isn't another option, really. IMO, the school probably shouldn't have run a second class of subject but equally we're the only school that offers it in a 20 mile radius, so that's also a hard decision to make.
I'm not happy about it, by the way, but it's not like there's someone else qualified waiting around to step in and teach that subject on something like a 0.1 timetable spread across 3-4 days a week...
I also think there's a lot of people on this thread acting like having no teachers in an inevitability- but it wasn't this way 5-10 years ago even. If serious action was taken on workload and pay, then we'd recruit, we'd retain more people for longer, and we'd maybe attract some skilled and experienced people back to the profession.