ChrisLy

In my opinion, there are a few things classes will teach/give you:

1. Power over words = different ways to twist/manuever concepts.
In classes, simply hearing concepts over and over, they will become ingrained. Once you have these concepts down by simple repetition of hearing them, you can add philosphies and twists to these concepts. Much like philosophy, you can then add steps of logic that you wouldn't otherwise be able to articulation.
For instance: Once you understand that a typographic concept exists, you have power over them. You can twist those concepts into ways you wouldn't by simply looking at them.
Add the fact that you can discuss these concepts with others in a class dedicated to these concepts and a sort of chemical reaction occurs.
This can happen outside of a class, but its much slower.

2. Competition.
The Internet and books generally only give you a view of great work. However, there is much to be said about looking at developing artists'/designers' work. Failure of bad designs teaches you things you wouldn't get out of seeing a successful, "fully developed" designer's work. Also, I believe there is a far bigger sense of urgency within a classroom. If you see a designer completely outdo you on the Internet, its easy to rationalize that they are far away and not within your competitive market. However, a classmate whose work you admire will seem far more "real."
It reminds me of how much faster you can run when you know you're being chased, as opposed to how fast you can run just by knowing some Olympian can achieve a certain time.

Again, both of these can be achieved on the Internet, but its far longer journey. There is a sense of urgency and push a class gives you that self-motivation simply won't.
This is all said, by the way, by a mostly self-taught designer (having only taken two design courses over the past 3 years).

I should also add that, in a class, you get the wisdom of a professor that has not only read books and learned the necessary concepts, but has gone through the process of THINKING about these concepts.