I would see if they could at least put him into 6th grade classes instead of 5th, either some or a total skip, since the school has both, if it were me. Of course, my DS10 does not have any athletic bent to him, so we don't have to worry about what grade-skipping does for sports issues.
I went looking for the chart that someone posted here a while back, but I can't seem to find it. Off the top of my head, a 255 spring math score is 99th percentile up to about 10th grade, I think.
My DS10 at 261 is going into either an 8th grade advanced math class in the fall, or if they don't have one, we're looking at 9th or an online class that has 9th. It's tough to decide if he should be in a class with high schoolers or not, so we'll have to play it by ear.
So your guy almost certainly (and it sounds like it from your gifted specialist's point of view) would do well to advance some in math. We don't worry so much about reading being advanced because the focus needs to be on math for DS10. We work around that for everything else. We are lucky that our school district's various pieces are all on the same campus here, K-12. If yours is spread apart, it becomes more difficult to get those advanced pieces at this age -- you may want to look at online classes, which some schools pay for and work with.
