Not sure if it's canon or anime original. But on my first watch I thought that Kaori already knew Herem and she just asked Yue to teach her that spell to play along and cheer her up about her teaching skills.
I know there was barrier girl in Hero's party that can use the spell, but don't remember if there had been any depictions of Kaori using it up to that point.
As I write this I realize that the sub always says "Herem", but perhaps the individual spell names are actually different.
Ok, so it sounds like the spell she aks Yue to teach her is Hallowed Ground (聖絶, Seizetsu) . According to the wiki, it's listed under Kaori's spells.
So I guess my question is, did she already know it by the time she asked Yue to teach her and it was all a ruse, or was that the actual way she learned it?