*Piprabbit, I have two DC at GS who have many friends at SM/comps.
THE SMs do really well for mid and lower ability DC but not always for those who are 11+ 'near misses', due to the quest for the magic 5 A*-C. In at least two SMs near us, this equates to 5Cs, even if the child is capable of much higher.
DC take gcses in year 9/10 and if they get the magic C, they are not allowed to retake, even if they get a B in year 9 (equates to A* in yr 11, I think) as friends DC did. So DC are getting lower grades than they should, at a locally-praised SM. It is especially annoying as at GS, she would have to resist as would not have met her personal target. So you need to ask when exams will be taken and school policy on retakes, hopefully this may be illegal under the new terminal exam system in the future? Also need to make sure you compare schools using '5 A- C Inc. Eng and Maths' to get a comparison of academic subjects.
It seems bizarre to get children who find exams harder to take them early, as surely they would get a higher grade and it would be better for their confidence than getting low grades in yr 9? After all, the DC with Es in year 9 should get Cs in yr 11 but the DC I know who got these marks thinks he failed. Is there any educational reason for doing this, apart from pursuit of C grades? One that benefits the child?
Benefits of GS, as mostly stated before: Less disruption, learning at pace suited to child, good discipline and, most importantly, high aspirations for all DC and high target grades. Excellent enrichment opportunities.
Downsides: need to want to work (but not overly hard!), May not be easy for those towards the bottom but many DC blossom when they get there so hard to tell in advance.
Sorry, long, but I feel this is unfair!