Although a (new) fan of this series by author, Jodi Taylor, my recommendation is to start with “Just One Damned Thing After Another” instead of this prequel. In fact I would recommend reading more than the first two books (which I have) before tackling this short story.
Although (mildly) interesting to discover how St Mary’s began, it was only mildly entertaining and interesting and I think it would pack a more powerful punch with more books under your belt (& a greater investment in the characters).