SOLVED. YA Thriller/Mystery. Teens all go to beach party the summer they turn 18, get picked off (killed) one by one right on their birthday. Spoilers ahead. [s]

[Emmohdee]

Spoilers ahead!

I remember reading this in the late 90s or early 2000s; had a early 90s/80s vibe to it but I suppose it could have been older. (English, USA). I'm pretty good at remember the books I read as a kid, or can at least use some google keywords to find them eventually, but this one's been stumping me for a while! I'd appreciate any help.

The premise is that a bunch of teens in the same town have graduated high school and all are turning 18 over the summer while they all go to some big beach party in a summer-camp-like setting. Before going away the protagonist finds odd things in the attic that suggest he (I think??) was adopted, or perhaps had another sibling at some point that had died early on.

At this beach camp out, they hang and there's some teen shenanigans and one by one they disappear, right around their 18th birthday. There's always a story spread around that they went back home, or off to college, etc. It gets more and more suspicious as the summer goes on, and eventually the surviving teens find the site of the ritual sacrifice where their parents are killing them in a ritual sacrifice for some sort of eternal-youth extension. They've all done this multiple times before with their previous kids, hence the weird stuff the protagonist found in the attic early on. Pretty sure it ends up with a few of the parents being killed at the end. One of the teens is a preacher's daughter and she's involved in the shenanigans somehow.

Thanks in advance!

[Andria]

Possibly Only Child ?

[Rainbowheart]

Still out there, Emmohdee?

Was Andria's suggestion your book?

[Emmohdee]

Yes, that's it, thanks so much!!