Oni Harami no Gakuen ~Sukumizu Shoujo Ishukan Ryoujoku Geki~

An academy located deep within the mountains and surrounded by nature. There are approximately 500 students at this originally all boys academy. This year it became co-ed and our heroines Rin and Mayu newly transfer to the school... only for something absolutely abnormal to occur! The students are turning into monsters and the school has transformed into a tentacle writhing cesspool of downright decadence... Behold and revel in the hard core tentacle play and situations! [From DLsite English]