This seems so unfair. 6s and 7s are not bad grades, and the school is taking away her opportunity to get qualifications she's worked hard for, as well as jeopardising her college offer and making her a less competitive applicant to other courses. I'd be furious, @LolaLouise .
She chose her 9 subjects, is on track to achieve her grades for her college offer, and the school is pulling this away now, when it's probably too late to do a crash course in another subject and the only real alternatives are parents spending more of their own money to make up for the opportunities taken away (sitting privately, extra tutors, etc.).
Individual science GCSEs might not be essential for STEM A-levels and degrees, but they can be very helpful. Students get more depth and breadth, so have a better foundation for subsequent studies, for applications, interviews, etc. And it's an opportunity to shine in a student's preferred subjects - if a student has an aptitude for science, why shouldn't they be able to get qualifications that reflect that, rather than having to do another subject that's less relevant and about which they're less passionate?
Having 8 GCSEs rather than 9 could put her and others at a disadvantage in general, too. When I sat GCSEs, the expectation was that 9 was the minimum for the courses that interested me, and I was aware I'd be competing with people who would be sitting 10 or more. My school could only offer 9, so I ended up doing one privately just to give myself the same number as other applicants would have. At this stage in the courses, though, it's probably too close to the exams for the OP's daughter to do that, though.
I'd be furious and I wouldn't let this drop without a fight, OP. The school is screwing your daughter over. It might be worth your daughter contacting the college and explaining what's happening - they might be able to adjust the offer made or find another option somewhere.