Looking for this hentai: The story starts with highschool girls

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Looking for this hentai:

The story starts with highschool girls getting fucked by dudes and getting graded for their sexual performances, it has people with clipboards and measurement instruments and all that stuff. The girls are done and the main girl is walking and talking with her friends and she sees her boyfriend in a class with a bunch of other dudes also getting graded.

The two have different "grade"-levels, so they aren't sexually compatible but they still like each other. She's self-conscious about that shit because it means that she can't get married with boyfriend.

It's an NTR story when this really gross fat dude after school tells the girl that he has the exact sex-grade level as her, so she loses her fucking mind and starts fucking the fat guy in front of her boyfriend. Pretty much ends with boyfriend crying and girl's friend shocked to see her turn into a slut. Also, at the end it explains that people who don't find a match pretty much end up at the sex-circus.