It's coming from the teacher. My DD has the same issue - she gets all worked up about grade-level tests, even though she's way ahead. The teacher says it's a big deal, that it'll be hard, that they need to worry.
For mine, having EXPLORE scores helped - "you did fe on an eighth grade test, so you'll do fine on this one." Also, for our state, the end of year testing has nothing to do with being promoted to the next grade, so I suggested she could get a zero with no consequence, and that helped, too.
