Do you need to go to school (college) to become an audio mastering engineer? The short answer is definitely "NO!" I never went to school, and I'll go head to head with any of the top audio mastering engineers in the world!
I don't have exact data on this, but I would say most audio mastering engineers do not have a college degree. The ones I know, most of them don't. And I know a lot of them.
And when I say school, I'm talking about a bachelor's degree from a major university. This is not a requirement.
But, you do need training. Books, videos, a 6 month to a year audio engineer trade school or course, and then either a hands on apprenticeship at a local studio, or even working at your own home studio using all the techniques you've learned. You have to put the time in and hone your craft.
How To Become A Great Mastering Engineer!
You become a great mastering engineer by learning the craft, practicing, and becoming great at audio mastering. This may sound like a silly answer, but these are the steps everyone takes.
The "learning the craft" part is where you have many options. You can go to a university, to an audio engineer trade school, take an online course, you could even learn from videos and books.
Of course, the best place to start is with my audio mastering book. 17 years of information wrapped up into 180 pages! You can read it here - Best Song Mastering Book.