A few interesting ways the story could go. What if the unable to be cut girl is actually quite old, and has been stuck as this indestructible high school for decades? Are there other cursed or supernatural humans or other beings out there, and the other girl is now being drawn into that world? Are their implications to falling in love with her, does killing her have consequences aside from the usual legal ones? Heck, does falling in love with her have consequences by itself? If uncuttable girl falls in love, would she not want to part from her lover?
Not sure if any of those will apply, but at least there's potential.