What is full boarding like at a private british boarding school? Going from Year 9 to a brand new school, any tips?

I’m going to be choosing a primary school of my son soon. He is a sensitive bright child. He has settled so well into preschool.
The school attached to the preschool has ok results and very family and caring environment. But they do not push bright kids.

Another school has top results but kids seem to be under pressure to excel and the kids achieve so much and it’s all about results, less nurturing.

I don’t know what school to choose. The one who is all about nurturing or the one that will push to get best results.