Why was she "chugging beers in a park" if she was a teacher. Is her school not reputable? Or is she so good at teaching that rules don't apply to her. Based on how much he learned in a short time I assume the latter.
Poor fella was the victim of NTR, huh. I certainly feel bad for what happened to him, but at least it allowed him to have this chance encounter as a result.
And he was actually moved to tears that she would give him such a great first experience, and as a result she decided to continue their 'lessons', huh. And in that ending, well, it looks like even if she had planned to stop after that time, fate simply decrees that they be together.
Maybe she can be the one to eventually mend his broken heart...Though in a way, I think she already has.
And it's likely that, after getting a hot 'n' heavy (and horny) taste of a "Sexy Onee-San", all memory of the first girl was obliterated from the mind of our young male protagonist; indeed, he had taken his first steps towards one day becoming A Man Of Culture.
And their meeting up again at the close only serves to confirm that as inevitable ... that Lucky Bastard (GRR)!
Once More, another gem from Bifidus-sensei is "APPROVED!"