Why specifically send him to a high school when college is better choice with women who have learned to pretty up than smelly hormonal teenagers, c'mon.
I mean, we have an entire NATION. Given the government at this point and the organized adventuring divisions etc, at bare minimum in those ranks alone they'd have a staggering number of hotties in their prime baby making years, ready to count the stains on the ceiling and do their adventuring duty for Japan.
...and thats before we consider the general public or adventuring school pools.
And as for bro refusing to bang girls to save himself for the MIA girlfriend... no problem, its Japan: Home to the Tenga and other advanced onahole options. Hook up the Ol' Semen Sucker Deluxe 9000 and have him make like it was an American school and shoot a ton of kids inside of it on the daily.
Like this isn't hard lol, why are they being so cagey about shit. All this 'we gotta send you to adventuring school' nonsense. Like whaaaaat? What is this over-complication of the issue?
How it should have gone: "Also, men are nearly extinct! We need you to do your part and save the human race... besides that you can clearly use magic and other abilities. You are our best hope of raising a generation to push back these dungeons..."
"Oh but I am a dolt and must preserve my virginity for missing plotpoint- I mean girl! Forget saving the world again!"
"Say LESS fam, I understand you are upset about Miss Ing Plotpoint. But we all gotta pitch in to save the country, yea? How about we compromise: We get our stud milking device, and every hot girl of your dreams to assist if needed. You do your part and unload a few loads to save Japan AND you'll get to preserve your virginity for the missing girl. Everyone goes home with a happy ending (heeyyyyoooooo!)"