The Witches of Adamas Vol. 5 (manga)

It's class trip time, and that means a fabulous three-day, two-night stay at a resort in the southern islands! Yukinari suspects all the creature comforts on offer are just too good to be true...and he's right! The real purpose of the trip is a global congregation of witches converging for festival, and the island paradise is a seduction-filled trap with no escape!