Mike Epps Stars in Trashy Student Video Project

5/22/2011 5:00 AM PDT

Mike Epps Stars In Trashy Student Video Project

"Hangover" star Mike Epps cares so much about L.A.'s sprawling garbage problem ... the comedian actually took part in a student film to help end the epidemic -- by doing a little littering of his own.

A couple HS students were filming in a parking lot outside some L.A. mall earlier this week -- and just their luck ... they ran into Mike Epps.

The two students struck while the iron was hot, asking Mike for his opinion on littering in L.A. -- and the actor responded by tossing a crumpled up piece of paper on the ground ... and saying, "Sometimes you don't want sh*t in your car."

Obviously, the guy was joking -- and he picked up his trash -- but Epps still insists, "The world is a ghetto."