A Butterfly Skimming the Water's Surface

This is a modern AU set in Kimetsu Gakuen:
Having fallen in the bloody battle at the Infinity Castle, Tomioka Giyuu is astonished to find himself reincarnated into a modern parallel world. In this reality, demons do not exist and his loved ones are alive and well; yet, aside from Sabito, his former comrades-in-arms are nowhere to be seen. Where did they all go?
The turning point comes when he accidentally discovers that his little butterfly is attending Kimetsu Private Academy as a high school student. Giyuu immediately devises a plan to win over his beloved-applying for a teaching position at the school just to stay by Shinobu's side.
Yet, will the strict campus rules ever allow the seeds of their forbidden affection to blossom? In this second chance at life, will he end up letting his beloved slip through his fingers once again...?