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Date: 1990-2007.
Genre: Thiller, horror.

A group of young people go on a camping trip to a woody-swampy area. It’s not necessarily an island but they do travel via motorboat to get there.
They are either in high school or college. With them, there’s a bespectacled guy whom they consider weird, he’s either their teacher or a guide, not sure.
Before the trip there is a scene in a library where they discuss the trip as a group. Also before the trip there’s a play in which one of the girls play Medea, a character from Greek mythology.
Her friends are in the audience and one of them asks another member of the audience something along the lines of “Why did Medea kill her children?”. (I tried subzin.com, didn’t find it.)
Once they’ve set up camp the dispappearances start. They all suspect the weird guy.
Towards the end of the movie – by chance – the main girl finds one of her girlfriends buried in the sand on the beach, with only her face and part of her neck visible, she appears garotted.
Then she bumps into a guy from her group and he claims that he also found a corpse.
Together they walk back to their camping site, where they see the weird guy sitting at the fire with his back to them. Convinced that he’s the murderer the guy starts yelling at him, as he is approaching him but when he gets there and grab him, he’s shocked to see that the weird guy’s body is lifeless (probably stabbed to death).
At the end of the movie 2 men arrive on a motorboat to rescue the survivors. One of the 2 men might have been the boyfriend of the main girl. It’s also possible that one of the 2 men was the actual murderer.

It’s not:
Do You Wanna Know a Secret? (2000)
Mindhunters (2004)
Hack! (2007)
Fear Island (2009)

Edited comment

Probably not the one you are looking for, but some aspects match your description, so I throw it in, you can never know:
“Hatchet” (2006)?
https://www.imdb.com/title/tt0422401/

Another suggestion, again, some aspects match:
“Safety in Numbers” (2006)?
https://www.imdb.com/title/tt0460556/

Thanks casspir! I’ve forwarded your suggestions and I’ll come back if there’s a reply.