I also agree with others, ask the school in detail about the grading system it uses @moond
I will say this tho – as a teacher it was clear what kind of grade a student was likely to get in my subject (MFL) in KS3. Yes, we hadn’t studied the topics but the linear nature of language learning means it is possible to assess possible GCSE grades in year 8.
I don’t think it’s especially helpful to say a specific grade, a 7 or 5 or whatever tho. I would err on the side of caution. But I think I could say in KS3 who might get (if they continued working as they were) a grade 7+ and whose best effort was likely to be a 4/5, and who was likely to gain < grade 4. It’s not set in stone of course – anyone can suddenly (or even gradually) start working harder or less hard. But it is a reasonable guide.
Same for English, maths, science, and to some extent subjects like PE, music, art, drama - where the evidence of years of learning, or alternatively keenness and commitment, are there in KS3; perhaps less so for hums subjects like history and geography that are so topic-based. But even there, skills or lack of will be evident.