This is a bit of a subjective question, there are quite a few variables. Things to take into consideration:
Film school offers coaching, and I believe this is the absolutely largest asset you will receive. You can learn most of what they teach in film school through online sources and books, but you won't have the constant coaching pushing you in the right direction. Another thing to remember is not everything on the internet is correct.
Through online and book tutorials, you'll learn the basics: Exposure, framing, rule of thirds, lighting, camera movements, equipment and DIY equipment, camera options, special effects, graphics, composition, color correction, etc.
I find these things more sparse and really do require some guidance and/or experience and practice: Lighting, proper mic booming, when/why to frame a certain way, when/why to use certain camera movements, what color correction is best for your film, avoiding poor choices such as jump cuts, etc.
I am fortunate enough to have a film school trained person in my group, and he has pointed out some of my huge mistakes that I didn't even think were mistakes. These things may have never been pointed out if it weren't for him. You can post up your videos for review and critique, but depending on the viewer, you may not get the information you really need to improve.
Summed up, film school is a great asset, but if you're the type that can learn things on your own very well, I wouldn't call it necessary. You have to remember that most don't ever make it as far as they want in the film industry, so if you do choose the schooling route, keep in mind it might never pay off. The term "starving artist" exists for a reason.
I encourage anyone who wants to make an attempt to make it on YouTube to go ahead, but keep in mind there is around 1,000,000 videos uploaded daily (based on my calculations derived from postings about the amount of footage uploaded to YouTube daily). Attaining viewership against the odds of 1,000,000 videos per day makes it nearly impossible to be noticed, even if your content is good. You need to know how to market your work (either through free means, or paying money for it to happen).