I guess one important question is... how long do you have before you need to send in your applications? It does seem incredibly unfair that schools want you to have so much knowledge in a subject that is so rarely taught in public high schools in this country, but they do want to be sure that you can survive when you graduate. They only have 4 years to get you to that point and there is SO much to learn.
You aren't in as bad shape as many students though. The beleaguered music professors I know describe kids who want to be music majors who don't play any instrument and can't read music. (These students do not get to be music majors). Since you've been taking piano, you're a step ahead of that crowd. Piano also gives you more career flexibility after graduation, since there are always people looking for accompanists, and it means you can write for piano and play the whole thing yourself, rather than having to hire other musicians or pester your scores into the hands of film directors or groups that play contemporary music to bring your compositions to an audience.
An online theory course is definitely a good idea, but be aware that the free ones are often for people with no music experience at all, so they start with teaching you to read music, which is not what you need and time is of the essence. Would your piano teacher be able and willing to add theory lessons or recommend someone else? Are there any semi-famous composers living near enough to you that you could take a private lesson with them once a week or every two weeks? (Google is your friend. Everyone has a website these days, and even famous composers have to live somewhere). If your high school doesn't teach AP Theory or composition, is there another school in your area that does? I'm sure that teacher would be happy to tell you what textbook they are using and you could buy it online. Does one of the local churches in your area have a kick-butt music program? Would their music director be willing to meet with you and look at your compositions? Can you find out what the Intro Theory textbook is at any of the schools you are applying to, and go ahead and start reading it now, and Google anything you don't understand?
How would you feel if you got into music school, but ended up making your living in some other musical way than as a film composer? How would you feel if you had to have 3 jobs?