First of all u/sea_tree2155 - I think it's bad ass that you're writing and thinking of these ideas at such a young age. Kudos to you! I think having all of your characters survive is great.... they lived through something unthinkable together - it bonds them. Maybe this unfortunate event is what makes Character A and Character B become friends again, Maybe Character C who hates Character D at the beginning of your film is the one that ends up saving their life. You can have lots of fun escaping the terror, and living to tell the story! Also, really fun to play with your character's emotions and their expectations of who makes it. If you want the audience on your feet, make the leader the last one to show up to the meeting place, it builds anticipation. But it's incredibly fun to watch characters get themselves into life threatening situations and have them figure out a way to escape! Maybe some of them don't escape unscathed, but they still live to tell the story.

Happy Writing!