I brought it up because OP is very young + low SSAT scores + says they are academically gifted but make constant errors on tests + says they have overcome a lot + says they only get along with older kids + says they are socially ahead (although I am not actually sure what that means).
All that together says to me a kid who might need some time to mature and learn how to socially fit into a boarding school. Staying the very youngest kid in the grade doesn’t, IMO, serve that purpose. Those things are all like tiny ding ding dings to me, something needs a little nudge back into a good path. Obviously, it’s the internet, we don’t really know anything about anyone and we are guessing wildly with all our “advice!”