How can you 100% affirm that?
Because i know what grooming is?
So do I and I think it's grooming?
You can't groom an adult!
So you're thinking wrong. The boy os already in the legal age to give consent, you may think the whole thing is strange, but that's your opinion. Also the seme even wait the uke completly finish his School before engaging in a sexual relationship
Also, It seems like people really don't know what grooming is (and i can 100% afirm that there is not such a thing in this manga)