I took 7 GCSEs a year early and didn’t find it a problem, although I did them in 1 year (started and finished the course in yr 10) so it is a slightly different situation. Some schools are now introducing a new accelerated curriculum, so it isn’t really very unusual to sit some, if not all of your GCSEs early. i personally think that it is a great idea, because what they currently teach in year 9 is a total waste of time. I missed almost all of yr 9, and still managed to go into year 10 at the normal age, with no gaps/ catch up problems whatsoever. GCSEs aren’t really that hard, and if your daughters are able then it shouldn’t be a problem for them at all. if they don’t get the grades that they want first time around, they can always take an extra year without having lost anytime at all.