They'll All Find Out That It's Inside Me! (Having sex with my classmate who's hiding under the covers)

"No… I can't let it show on my face that I'm… Hnng!!" A group of three boys set off on a completely sex-free hot spring vacation... Or at least, they were supposed to. But in an unexpected twist, they run into three girls from their very own class after checking into the hotel! After panicking and hiding under the covers, one of the boys has his first experience touching a female body in a near-suffocating fashion… when suddenly, it slips inside!? From between the gaps in their robes, the erotic juices won't stop flowing! And when things escalate to a drunken party game with the girls, just what will happen…!? "I can't move an inch… or it will go even further inside!!"

(Source: Eromangajima, translated)

Note: Released in 108 episodes and sold as 36 chapters.