There are two professional schools in the country, to my knowledge: United Bicycle Institute in Ashland, OR, and Barnett Bicycle Institute in Colorado Springs. I believe Park Tool has a curriculum that is taught in various bike shops around the country, but not of the same professional level. I did the UBI 2-week course in 2008. Students range in age from 19 to 65 and in experience from nothing to several years in a bike shop. Those with no experience struggled, and those with moderate experience had a great time and really added to their knowledge base.

What is your background and experience? Are you looking for full time employment as a mechanic, or will this be a side-line or part-time career? If you're starting from ground-zero, taking a Park Tool course or whatever you can find locally would be a good start. You may find a bike shop willing to hire you and train you, but some shops only hire experienced mechanics.