There are lots of different ways to get your start as a director, but the most important thing is to actually go out and make films. If you go to film school, you should still be making films - it's not a choice between going to film school or directing a movie.
Here's the disadvantages of starting on your own:
Starting on your own you may never be "discovered" or your film might never been seen by the right people to get you your next job. You also probably won't have as many connections to people who can help you in your career as you would if you had go to school with other filmmakers. Going this route you could end up spending lots of time and money making your own films that never end up being seen by anyone - or you could be the next Ben Afflick and Matt Damon, but those stories are few and far in-between.
One major advantage of being in a film program is access to equipment that you otherwise wouldn't be able to afford, and you'll get training in things like lighting, editing, sound design, etc that would take much longer to learn on your own.
Disadvantages of Film School:
Most film schools take 2-4 years and will put a dent in your wallet (some more than others). Which is something to consider since a degree in film isn't usually necessary to get work in the entertainment industry, although it doesn't hurt.
Another big disadvantage is that film school program are rarely specialized enough for you to get work doing anything other than PA stuff (production assistant, lowest rung on the totem poll). Usually in a film program you learn a little bit of everything - screen writing, producing, directing, film history, cinematography, editing - but you don't learn enough of anything to actually get a job doing it.
Pros, Starting on your own:
- You'll save money on school tuition
- You can spend all your time working on film, no other classes that you don't want/need
Pros, Film School:
- Access to equipment
- Networking with your peers and staff that will help you to advance your career
- Training in things that would otherwise take you much longer to learn on your own
Cons, Starting on your own:
- You'll have to rent/buy your own equipment, insurance, etc
- It may take longer for you to learn certain aspects of film making
- It's harder to meet the right people and make connections (vs. film school)
Cons, Film School:
- Tuition Cost
- Usually you don't get specialized training, so you have to start on the bottom of the totem poll and work your way up
So, with all that in mind - I recommend learning to do something specialized, particularly in post production, so you can get the best of both worlds and be on a faster path to becoming a director.
All my friends from film school who went into film editing moved up the ladder much faster than others who studied directing. In fact, many of my editors friends are already moving in to directing.
It used to be that the only way to learn film editing was to be an apprentice under someone willing to teach you, but now there is a school in Los Angeles called Video Symphony that has a 1-year program in TV & Film Editing. Many of my friends got training there and are doing quite well. Basically you get all the advantages of film school without the 4 years of paying high tuition and you learn a skill that can propel you into directing.
Check out these articles about editors that moving into directing: vsarchive.com/articles
Hope that helps!