SOLVED. YA thriller from the 90s, teens trapped on island, murders [s]

[Maureen]

It's been 25 years since I read this, so the plot details I have are very sketchy and unreliable. It's a YA novel that I think I read in the early nineties, so I think it's possible the book might even have been written in the eighties. The cover was that classic hand-painted art style of the era, and I feel like it might have been an image of a school bus from behind, indicating a school trip. Not certain about the bus, though. That could have been from another book.

It's first-person POV with a female main character. She and approximately six friends travel to an island, but I think a storm blows up and traps them there. There might have been an adult with them at first, but they either got separated or something happened to the adult. Anyway, it's teens alone on the island, and one-by-one the teens start being murdered.

Major spoilers ahead about the plot ending!
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In the end, the murderer turns out to be...
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the POV female. Which blew my mind as a kid. The only detail I can really remember other than this is that someone gets killed with a flare gun. Do any of my 30-something contemporaries remember this book? TIA!!

[Andria]

Class Trip?

[Maureen]

Want to add: I don't think it was written by any of the major players. RL Stine, Lois Duncan, Christopher Pike, etc. I got the sense that it was a unique one-off, which was unfortunate because I really loved it.

[Andria]

Hi Maureen, did you see my suggestion, above?

If that is the one, there is actually a sequel: Class Trip II

[Andria]

Maureen, did you move this to solved? Just want to confirm we have the correct title before we officially close the thread.

[Maureen]

I did see it, and I moved it to solved. I responded as well, but for some reason I don't see my comment. Thanks so much for your help!

[Andria]

You're welcome!