In an ideal world, kids will always get teachers who have the drive and resources to offer work that challenge s them. I know both dh and I were left unchallenged by school (some years), and either pretended to not get it, became class clown, disruptive or just skeptical of education in order to amuse ourselves. Other years, we had brilliant teachers.

Just one thing that is coming in to working out what to do. All my kids are born at start of the year, so we will have to do this a few times.

This was our concern too with DD1 as DH had similar issues at school, not challenged, but I've not once heard a parent say the regret sending a child older, only those who wished they'd started them later.

The resources are also limited when it comes to kids who struggle, I've got friends who's kids are struggling with reading, there is only do much more the school can offer/do.

ETA the other thing you will need to consider hot is that if your DD does another year of 4 yr old, it won't be funded, you'll have to pay the full fees out of pocket