In my opinion (u may disagree with me but hear me out)
One, He's in highschool so he is technically somewhere between 17 or 18. Two, grooming is a form of manipulative abuse of an adult towards a child, where here no one is manipulated or abused. Three, both the teacher and the student waited to official date and being intimate when the student graduated.
I will consider this grooming IF the teacher truly blackmailed him into sleeping with him like the other yaoi manga we read and since we saw the part that the teacher helped him when he was naked and yet the teacher never tried anything out of respect since he is still young and trying to understand his sexyal orientation.
So when the student finally graduated and is now an adult, only then did the teacher became intimate and affectionate with him. So No, not grooming. Just an adult respecting the boundary and a student understanding his sexual orientation, them being respectful to one another.

would this count as grooming?