Are you looking to do sci fi like weird twilight zone stuff??
Just spit balling, trying to think of something that would be good for a group of people to do, giving everyone different parts.
Ok, something that could take place in a highschool. Since its a short film and you dont have allot of time to be really elaborate you could establish "A" typical characters in it. Jock, Nerd ect. But also establish a more unique need with someone, or a group of people. You could tackle one of today's current issues like, increasing teen pregnancy or something people can relate to since it's such a short film.
Then have a group of new kids, maybe two? join up with the school. Have the kids be a little strange, odd habbits, odd figures of speach, but odly enough they fit in really well, seem to know things about people immediately, intimate details of their lives, but not in a stalker sense. They end up being not from either this world or time, you take your pick. Could be done really easily "twilight zone" like at the end have them dissapear into another dimention or time and if you get creative you wouldn't have to do major special effects.
Anywho, the new kids come in and change the lives of the students in the school in a way that no one else can. You could have a voice over or something at the end about why they would do this. They could be goodwill anbasitors from another dimention trying to help the youth create a better tomorrow. Or they could be ghosts or something trying to pay off their debts. A thousand possibilities here.
But I don't know maybe it's stupid and already been kinda done. Just hard to think of something really origional that can be done in such a short span of film. You probably have what 5 to 6 script pages you can work from?
Otherwise... I don't know, you could do a spin off of "a christmas carol" or something. but have it done modern day by someone who doesn't believe in that stuff. Could make it kinda funny by having the ghosts be kinda rediculious and teach the person something totally unrealted to the actual "christmas carol" lesson.
You cold tackle another bigger issue this way, like overeating or something. You could have the ghost of beer belly, ghost of flatulence and ghost of transfats or something. I dunno.
This probably wasn't helpful but maybe you can get someone more creative than me to make it work.
Good luck!!!