SPEAKING FROM EXPERIENCE:
My DD went to a v competitive grammar where you had to get a 7 in a subject to continue at A Level. She got a 6 in one of her chosen 3. School said she couldn't do it at A Level, so she had to pick another subject - chose English Literature (where she had done exceptionally well at GCSE but hadn't really previously considered).
When we got a breakdown of the paper/section scores for her level 6 subject, one of them had been recorded as 0. Ordered a remark and it was a complete clerical/admin error and she was immediately bumped up to a 7 as her marks in that part hadn't been counted so could switch. However, by then she was enjoying English so much she opted to stick with it.
She's now just finished her first year studying English Literature at an RG uni and is happy as a pig in shit.
So - don't be afraid to challenge grades if you think they're wrong, even if you're not a couple of marks away from boundaries. And don't be afraid to follow a different path than the one you thought they would. I could kiss the person who recorded that 0!!!