Of course, there will always be exceptions, but it seems reasonable that scores might drop the first year students move up to a higher grade level on the SOL. My child also went from "Advanced" to "Proficient," but it was only about a 20-point drop—which feels somewhat arbitrary.
I’d be more concerned if a downward trend continued in the future, but I’m not going to split hairs over it during the 5th grade year, when they’re making that initial jump.