Students who were very anxious on test day scored 12 percentile points lower than students who were less anxious. That means the anxious students scored about half a grade below. Therefore, less anxious students scored about half a grade higher than anxious students.
Reducing stress can improve your test scores so be sure to take a few extra deep breaths and stay positive.
Ray Hembree (1988). Correlates, Causes, Effects and Treatment of Test Anxiety. Review of Educational Research, 58 1, 47-77.