I think there are a lot of things to be considered to answer this question completely.
If it's just about learning about the business and the process of filmmaking (the "craft", if you will) then a filmschool
could be useful but is not necessary. A film school usually offers a variety of equipment
so you can try out different cameras, lights, etc. and find out what you like best, figuring that out alone can be very expensive.
Also, depending on the film school, you might be able to get in touch with professional
filmmakers, after all it's always about knowing the right people.
You can of course get to know them while making your own movies or helping others
with theirs, so even there a film school is not really "necessary".
It also depends on what kind of films you want to make.
If you spend a lot of time thinking about the terms "art" and film theory,
then (at least that's what I assume) a film school might be the place where you can
deepen your knowlede and discuss your views with other people.
If you just want to entertain with your films, then this aspect is less important.
In the end, it depends on you, how much money or energy you have and your interests.