this particular movie, possibly only a low-budget short as part of an anthology (the kind shown late at night/early morning on C4, E4, BBC2 et al) if you've ever watched the house that spilled blood, the house that bled, tales from the crypt, vault of horror et al you'll get what im on about..
starts off with a bunch of old people crowded round a Ouija board (i think, or maybe it was a crystal ball - something mystical anyhoo) - and they make some sort of pact/agreement to keep their teenage/young adult offspring safe etc.
this manifests itself when the kids discover that they will not die and cannot die if they believe they cannot, the only part from then on is they all do some sort of ritual, which ends witht hem riding their motorcycles thru a brick wall (which ordinarily would have killed them). except, the girlfriend "misses" the brick wall as she did not enter into the ritual thing (which her bf then confronts her about).
the parents then "undo" the spell/curse/voodoo/thingy and all the kids (except the girl) go into a kind of "fast forward death" while they are standing up and their bodies basically rot and turn to petrified matter in seconds. not particularly gory as it was produced pre-naughties - ie only the most basic visual effects (ie quick-time, obv not the apple software) as was available at the time.