@Readingisgood in simplistic terms they mark all the papers for a subject and then look at where the average falls, that becomes grade 4 which in old money is a low grade C, a 5 is a high grade C. Then they work out everything from there up to a grade 7, or an A. To work out the grade 9 they apply an algorithm from grade 7 upwards and then the grade 8 falls in the middle I believe.
Grade boundaries for past exams can be found online. To put this in real terms the grade boundaries move with the cohort, the students sitting the exams. A grade 6 one year can be a grade 5 the next with the same marks because more people have performed better and got higher marks.
Some boundaries are tight, History for mine was out of 210 total
125 grade 6
140 grade 7
156 grade 8
173 grade 9
This is calculated over 3 papers worth 80, 80 and 50.
As well as that grade curve the other thing to realise too is that it is an unfair playing field, state school children are competing against parents who have paid out tens of thousands of pounds to ensure their child has a better educational experience in private schools. I don't begrudge them because if I could have afforded it I would too.
How I helped my children, talked about post GCSEs, the grade curve, showed them jobs and pay scales, explained housing costs because minimum wage sounds like a lot of money when you live with your parents. It doesn't sound as great when you look at rental costs, all the utilities, broadband, insurance, food and travel costs. School also covered this on a PSHE day. Showed them what mortgage you could get on a teacher's salary. What that buys you locally - fuck all if buying alone because we moved here for an outstanding primary, secondary and sixth form. House prices reflect that, along with great transport links and local amenities.
I also looked on here a few years ahead of where my child was so that things like revision, A levels, uni choices etc were not a surprise, it was something I had researched and knew to help guide my children through it. I had very hands off parents, Dh had much more involved and supportive parents which is why he went to an incredible uni, great grades all the way through and a very well paid graduate job.
As the wonderful @clary says (who personally helped me through MN help my child get a grade 8 in GCSE French) her DD recommends flashcards at the time they do the work, which is what we did. I also told mine that there was no way I wasn't going to be on their case about homework etc because free education is not to be underestimated. There are no widespread resits, only maths and English language if you get a 3. If they hate maths try to pass it first time otherwise you will be doing it for longer. It would be difficult to have another go at Geography because you got a lower grade. I know women in their 30s having to resit maths GCSE to get the job/promotion they want.
Having said all of that you can still get children who won't/don't engage with any of it.