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.
It felt very heavily like grooming to me. He is insecure of his sexuality & really wants a relationship, & wants to be loved. If he had graduated high school & they met again when he finished or almost finished collage that wld be different. However the ML teases & flirts with the MC throughout the story, which wld encourage an adolescents feelings.
Hmmm. If the teacher is around 20 to 23 and he is 17.. okay I can ignore. However if this teacher is pass 25. Then i would say yes, its about power imbalance. Because one has more experience and knowledge.
I dont think age gaps are inherently bad or always predatory but it is common that predation is more likely with this.

would this count as grooming?