The insistence on frequent regular testing, as do OfSted inspection criteria, leads to syllabi mapped out for each lesson of the entire year, and is a huge problem.
You also need to bear in mind that, in some subjects like physics, the teachers taking the lesson cannot really teach the subject, because they do not fully understand the material themselves.
All lessons inevitably become 'content delivery', rather than teaching.
The teacher presents knowledge, rather than teaches, so students become demotivated and also learn little of the fundamental principles, especially those with associated misconceptions.
Students therefore are unable to pass tests requiring synthesis of fundamental principles, so firstly exams are made more predictable. This enables teachers to coach students how to answer questions they do not understand. However this coaching reduces teaching time, so it becomes necessary to reduce content in order for student grades to continue to improve. However, the removal of difficult content (usually the first victim), both demotivates the brighter students (and the brighter teachers), and often removes key links between areas of study which, if properly taught, would actually reduce student workload. Next, in order to claim that the exams are just as difficult, the language is deliberately made more complex, thus not actually testing the subject but English comprehension. This is of course a nightmare for students without highly developed vocabulary, such as science types, foreign students....etc. With students demotivated, the Government then decides to 'sex up' the syllabi by asking questions to which the answer is "iPod', 'The internet' or 'Helicopter surveillance like on Mission Impossible'. However, the dinosaurs in both the DfE and the exam boards don't actually understand any of that, so you now also get stupid questions with all wrong answers. You can't teach how to answer these because the science is actually wrong, so now you have to spend even more time drilling the students with practice exams....etc.
This summarises the decline in all UK exams since about 2000, and many since 1983.
The emphasis on practice exams comes because the school leadership place the highest priority on that. It is not exclusive to schools with low experience teachers. The same is true, from my own experience, of some high performing grammar schools.
I would do practice exams for 2 or 3 weeks maximum before GCSE and A levels, with only one mid-year exam (which had 1 lesson's revision & practice). This leaves much more time for teaching properly. Furthermore, I wrote syllabi where the content was delivered in only 2/3 of the teaching time, leaving 1/3 for recaps, and teacher/student interest areas to be developed. I had suggested additional work, carefully pre-planned, for either new teachers to gain confidence or for senior teachers who were in a hurry.
I checked on where my teachers had got to once per term. There was no insistance on sticking to any kind of calendar, though one was provided to guide new teachers.
On some occasions, I did not set 'standard' (i.e. marks out of 10) homework for half a year.
The results were, pretty consistently, among the best in the UK for all abilities of students, at 4 different schools. It also produced exceptionally high proportions of students studying the subject beyond school, and they then achieved exceptionally well both at University and in employment. We set up feedback loops to ensure we knew how they did subsequently, and modified our teaching in order to sustain this.
The system, of course, absolutely hated us.