SUMMARY: The story starts with a group of four friends in one of those setting were practical sex education is part of the school system. The protagonist has a crush on Mei, but when the day for the sexual relations class finally arrives as member of the health committee she's chosen together with the other guy (Shintaro) as the couple for the first practical demonstration.
From this day forward Shintaro keeps having his way with Mei as his partner at every ocasion, while the other girl and childhood friend of the protagonist (Minami) tries her best to make him notice that she likes him, but...will she manage before it is too late?
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Fisrt compilation of the Sex during School's Lessons published by the artist in Pixiv:
This series is kinda confusing. We get an ending here with MC impregnating his CF, but the following chapters we go back to point 0 and get a truck load of new characters , I dont know what is going on anymore.
at first of story, MC(fat boy) likes black hair girl. but she and grey hair boy made a pair because they were menbers of school health committee. at middle ,grey boy suggested swapping, when grey boy and ponytail girl are playing, MC noticed he likes ponytail girl.
Probably the confusion comes because the author initially didn't expect to expand the series so much, hence the inclusion of the time jump to the Outdoors School trip and the pregnancy epilogue at the end of this chapter.
Reading the notes of chapter 3 (which covers in detail the Outdoors School) the only thing that he had clear is that the protagonist and Minami were going to end together. For the time being you can ignore the epilogue here, because from the point of view of the timeline Chapter 2 takes place right after the first weeks at the beginning of this part.
I'll try to add notes on the following chapters to clarify that.