Would you be able to explain to your teachers that
1. Your mock grades were only an outlier to your consistently strong results
2 even if your 80% class test scores are only “average” in comparison to the rest of the class, this is literally just 1 school and a level grade boundaries are based on national performance, rather than individual grade boundaries for different schools so it is just very unfair for teachers to judge your predicted grades on one case
3 they are predicted grades and over a year students can and do drastically improve, and this is on top of the fact you’ve already been achieving really high results
4 MOST IMPORTANTLY, the reason having good predicted grades that actually reflect what you are likely to achieve + your skills + hard work is because it means you have to apply to different and less competitive unis than you want to, and may even have to take an additionally year of your life of before uni - and neither of these are if your own choice. This is not just bad for you but also for your school (less ppl going to top unis).
The main thing you should stress to your teachers is that these are very unfair especially as you have had to deal with poor circumstances and as the ppl who know you (in contra to uni admissions staff) they should know your true capabilities and what is best for you. And also show evidence that you are very committed to getting As/A*s (eg asking advice, keeping doing well in tests, maybe talking about doing STEM olympiads, that you want to apply to unis like Imperial and Nottingham and therefore are ambitious and school staff should not stop you from achieving your dreams).
(This is just from my understanding of the thread, pls correct me if I misinterpreted anything)
I hope this helps and things go well for you