Honestly, I'd keep them home til Sept.
In your shoes (assuming this is first child you've adopted) I can see why you might think that that's nuts (and that they'll need their routine and you'll need your space and so on) and also that it's hard to argue with the SWs plan, but in all honesty, if placement is this month then you're looking at keeping them home til Easter anyway, which means starting them in school for just one term, then there's another transition into the holidays to manage and then another back to school and new class/teacher/classmates. That's a lot for a little kid who will still be learning how to cope with home.
Add to that that they'll learn nothing anyway if they're stressed and that they'd be a million times better off bonding with you than at school.
What are your thoughts? What made you post? Do you think it's short or do you just not know?
To give some context, I have two AC, one other side of your LO age-wise. Both seem equally bright. DC1 started school 10 months post placement and has struggled since day 1. DC2 took to it easily. How much I'd love now to be able to go back and keep DC1 home another year.
If you're nervous about raising it with SWs you can smile and nod for now and raise it post-placement when you have more say. (Or feel like you have more say at any rate!)
You could also consider having them repeat the year but again, it might be worth waiting til you have met/know your LO.