What are your internships and where?
How many films have you made so far?
When DD applied in 2011, only USC and NYU required a film submission. LMU might require one now. Chapman requires a film whereby you kind of show them who you are via a 2 min film.
You do not need an extensive portfolio, you just need one good film. (I forgot the length…under 5 min or under 3 min). The benefit of doing a summer film program is that you will have 1 more film to evaluate when you are trying to decide which film to submit. What DD liked about her summer film program was that they got to audition actors from the acting program, so that she felt that at least she had better quality actors in her film than in her HS or leisure time films. She did not submit that film, tho, and instead submitted a film that she made in her leisure time So you could make more films during the school year. Or if your HS has film classes, you could make films during the school year.
Besides a creative resume that @jtmoney mentioned, a school like USC will require also either a script or a short story which will be told visually, so here is an area where your writing skills might shine. And your creative resume does not need to include only films. It can include anything artistic or storytelling, such as your creative writing, theater, acting, drawing, photography, etc