Back when the season started, RAS posted the above picture as a tease, and no one actually died. But it seems he was doing the long con!
Now to narrow it down, some time ago a loose lipped leaker told their tumblr friends that one of the teenagers is going to die during the musical episode. This already eliminates Valerie and Melody since neither actress was available to film the episode. Also, the regulars are safe (though in my dreams it would be Jughead) so breathe a sigh of relief that Josie will live to be wasted some more.
And if you need picture reassurance, here are some of the regulars thanks to Marisol's IG stories of filming.
Ethel's also safe so that we don't have to go through another iteration of #justiceforBarb
It's OP's personal belief that the likes of Chuck, Dilton, Toni, Sweet Pea, and Fangs will also be fine because they're not in the wedding photo. Though of course OP can be massively wrong. Like for example that Dilton's actor is going to school now so either say hello to a third Dilton or goodbye to the current one.
But going with the wedding photo assumption would leave us with Reggie, Moose, and Midge. Charles was recently seen on set so that could seemingly take him out of the running though for all we know it was to film his own character's funeral.
Would the writers be cheap enough to put Moose and Midge through a near-death experience fakeout just to actually kill them off later? I think we all know the answer to that
We'll all find out in April though! Unless of course some other butter fingers spills the beans