Getting a 6 in GCSE is not difficult at all and you should easily achieve it if you don't make too many mistakes. Examiners want to assess a basic level of competence on the GCSE exam, but they also want to distinguish between candidates who can apply their previous knowledge in unknown contexts and those who cannot. A Grade 5 is considered a “strong pass” and is equivalent to a high C and a low B in the previous grade system. However, getting a 6 in your GCSE won't be enough to impress colleges and universities, as it only shows that you've done the bare minimum to pass your GCSE subjects.
Science subjects such as Biology, Chemistry, and Physics (along with any combined version) are some of the easiest subjects to pass at GCSE. Even if you don't study too hard throughout the year and only increase your level as you approach your exams, getting a 6 in your GCSE shouldn't be too difficult. It's important to note that a Grade 3 in Year 7 is not fully equivalent to a GCSE grade. This means that it would be common for most Year 7 students to get a 6 in their GCSEs.
In conclusion, getting a 6 in your GCSE is not difficult and it's considered a good grade. However, it won't be enough to impress colleges and universities, as they expect higher grades from their applicants. Science subjects are some of the easiest to pass at GCSE, so if you study hard enough you should be able to get a 6 without too much difficulty.