Honestly, I think an assembly with a real, live EMT from the community telling them stories about what the EMT's seen, possibly with videos for illustration and graphic impact, would work better than the shock tactics.
Tell them what's happened on the actual roads they travel everyday, at locations they know, (probably not to people they know because of confidentiality rules) stuff they can visualize.
Real stuff told to them by someone who was actually there, dealing with the aftermath, is probably going to have as much or more impact than a made-up scenario that they can write off as 'more adult lies'.
Because, yeah, hearing that a classmate died may make you feel horrible, but once you know the truth, the feeling of betrayal sets in.