there is no clear answer. it depends on your level of knowledge going into it. for example, a friend of mine, as a junior in high school, had films premiereing in festivals around the country, and one got an indie distribution deal. he chose to not go to film school, and obviously someone with that much natural talent and who knows that much isn't going to be missing as much as someone who doesn't know anything about movies.
i also unfortunately know people who THINK they're good, but aren't. it's a harsh world, so i don't try to sugarcoat anything. this is basically what i have to say. if your movies, BEFORE film school, don't look a professional as an actual movie... you're not there. i know people who think they can write or direct or shoot. and maybe they can do some cool stuff. but if they aren't THE BEST at it before film school, they need to go. otherwise, they're screwing themselves out of education they obviously need.
so that's just the way i see it. many people get started really young in film and don't need help. others do. if you don't know exactly what it takes to make a great film, and if you haven't done it yourself yet, go to film school. it's as simple as that. those who need education should go to film school. those who know how to make movies do not have to.