Sorry, I don't have as much time as I need to fully respond but the biggest thing that jumps out at me is...

How would your school horizontally enrich a child who is working several years (or even a year) above grade level? For example, when my DS8 was in first grade he was working about 3 year levels ahead in maths and literacy... how could horizontal enrichment of first grade material have been enough? He got both extended both ways, and even with that we was capable of learning even more at school than he did!

I would really insist the school explains this you before you start.