Interesting question!
Not sure there is an easy answer... but let's try.
I've read "Study of Orchestration", I assume you mean Adler. I had guidance from a teacher, and yeah, it helped, but for me the real key was listening as I read. I think I would have gained some insight without a teacher. It isn't really "out there", it's a pretty basic (well it gets advanced) framework for orchestrating a melody.
There are books that are out there - and a book on modalogy might qualify. Once again I'm not sure everyone must have a teacher to guide them, but it certainly doesn't hurt.
One of the biggest advantages of a classroom setting, formal or otherwise, is that it helps you focus. There are theory books on my shelves that I haven't finished because I didn't find them all that interesting, and no one made me<G>. I've revisited some of them over the years, and sometimes I've discovered I probably should have been a little more disciplined.
My field of study was physics (long story skipped), but I didn't even figure that out till the end of the first year of 'pre-med' studies. In order to try to catch up I tried to read the general physics textbook over the summer. Some folks might have pulled that off, but I needed a teacher to (a) keep me on track, and (b) explain concepts that weren't entirely obvious in the text. I've got a book on quantum physics and another on electricity and magnetism on my shelf. There are, I'm sure, people who could teach themselves with these books, again I wasn't one of them.
I'd guess the number of folks that can learn by themselves from any of those books is very small.
I'd further guess that the number of folks that could teach themselves elementary harmony or basic orchestration is orders of magnitude larger. Among other things the required experiments are more accessible, and I think a little more fun. (Not a lot more fun mind you, some of those physics experiments were a blast!)
So my advice is go ahead, dig in, give it a shot. If you get something out of it you are ahead of the game. BUT, since you have the opportunity by all means work with the teachers too!