While I’m not sure I could argue that either of these words are vital to the high school student, that’s not necessarily the point of the English Usage worksheet on differentiating between the use of religiosity and piety when discussing the concepts the two nouns represent. This document, rather, seeks to help students make usage decisions based on the subtle differences between two words like these.
If you find typos in this document, I would appreciate a notification. And, as always, if you find this material useful in your practice, I would be grateful to hear what you think of it. I seek your peer review.