Her academic performance had been going downhill throughout the semester, so failing the final exam was expected.

going 是动词 go 的现在分词形式，表示"进行、进展"的状态。 downhill 由 down（向下）和 hill（山坡）组成，字面意思是"下坡"，引申为"走下坡路、变差"。 going downhill 这个短语用来形容情况逐渐恶化或表现持续下降。