Are you for real? Nurse knew a minor student since the beginning of high school and “waited” for her until she was an “adult". Please don't call that sht professionalism, the system education really failed you.
Waiting until someone is an adult is not grooming. Grooming is approaching and doing various activities that only an adult knows how to do well to increase the minor's attraction toward you. It's in the name. They're "grooming" them to be in love with them. The nurse explicitly tried to avoid doing that.
Yeah, grooming involves intent, or at least some manner of negligence that involves leading the minor on. I can't remember their every interaction, so I can't recall if she ever let slip that she planned to cave after graduation. I know we knew it, but I don't recall if she ever let on to Mizuki that it was in the cards. The actual appropriate thing to do is to shut it down hard and write it off in your own mind, though, and she clearly never did that. So while I wouldn't call it "grooming", it was at least inappropriate.
But, again, a thing happening in a story does not mean that it is depicting that thing as a positive example for adults to follow, and "My hot teacher is actually into me" is a very reasonable sort of fantasy wish fulfillment for the teens this manga is actually written for.