Summer Days 17 by Mayu Shinju
Seventeen-year-old Ishimori finds herself alone with three boys for a summer trip. This is a tame oneshot by Mayu Shinjou. What do you expect ? It's a oneshot after all. XD There's no need for the pics of bishies because the mangaka always have the same style for the main man.
Thanks Lolly for the tip.