Honestly, from seeing what we study and seeing what's on the internet, I could be self taught, but I don't have the discipline for it. (Sounds about right from a maplestory addict?)
and it's great being around other people who have the same interest/hobby as you.
I think people made lots of good points already but just want to add that school has placements which can give you ties within the industry making it much easier to secure a job once or even before you graduate. It also builds your portfolio (in much more ways then I think you would self-taught, we did lots of work with other companys/events/etc. And something like graphic design has way to many areas that people usually just focus on one but it's much broader then that)
At the end of the day for most fields (and especially Graphic Design) you can never stop learning even after you graduate, especially because of how quickly technology has and will advance. But I think it's important to lay that groundwork first and school is really good for that.
Anyways sorry for getting slightly off-topic. I didn't know this was going on, I'll give it a go