Your mileage may vary.
Barbering is an intriguing occupation: in prehistory it was witch-doctoring, a cross between spiritual healing and surgery. The ancients believed that cutting hair and plaiting hair were akin to relieving humors and binding spirits in or out. As a result, and also because barbers accompanied the monks in the Crusades (who, as the only literate and educated folks, tended to be the doctors) because of their need for tonsure - barbering was eventually classified as a subsection of surgery. At one point the barbers and surgeons shared training facilities and the guy who sawed your rotting limb off often also cut your hair.
With the advent of germ theory barbering came under the crosshairs as it became uncomfortably obvious that a vector of disease transmission was the re-use of a razor across multiple clients. (Most modern men don't realize that through most of human history you didn't remove your own facial hair, but had another do it.) Barbering by that time having formed guilds, in the interests of public health and also in the interests of people not being scared away by the profession, they established standards, colleges, and licensing requirements.
For many parts of the US that DO have licensing requirements, they tend to be in the range of 1500 hours of training by a master barber or a licensed college, and following that a written exam and a practical exam in which you cut the hair of and shave a model (or mannequin), demonstrate the use of perm rods and curl rods, facial massage, and so forth. Successful performance in the exam grants you a barbering license in your home state.
Considering that apprenticing is a hassle, your mileage may vary in terms of your experience and that taking on an apprentice brings the inspectors around more often - there are dedicated barbering colleges designed to teach all the theory involved (usually from the Milady's Barbering Textbook) as well as the practical - demonstrating on and having students practice on multiple types of hair and clients. In theory.
Barbering colleges also vary. Some are accredited, or attached to actual community colleges - so they have all the standards of colleges: they require proof of high school graduation, electives, a very formal classroom structure, and fees in the $15,000 and up range. Because a barber doesn't need a high school education in most places, some colleges choose to remain licensed but not accredited, meaning that they can offer courses to more people at a significantly discounted rate. It also means that you are at the mercy of just about any standards possible (within legal and licensing limits) - and your mileage can most certainly vary.
The average school looks like your common and garden hair salon, with the exception that there are classrooms out back. The equipment takes a beating, so chances are your barber student will be dealing with idiosyncratic chairs, towels that have been laundered MANY times, and so forth. But it's all serviceable, even if the lighting leaves something to be desired.
Typically, they work on the premise of a junior and then a senior status, assigning you more duties as your knowledge increases. You almost always start with theory classes, and have theory classes until you have worked through the entire book.
The textbook takes you through the history of barbering and the basics of anatomy, physiology, chemistry and electricity. You will also learn how the tools work: scissors, clippers and razors as well as esoteric devices for light therapy and electrical therapy - how to take care of them, and how to use them. Once you have demonstrated theoretical knowledge of that and the difference between disinfection, sanitation and sterilization, you typically take on your first clients. You are also expected to have a basic knowledge of diseases of the head and scalp: ringworm, lice, and so forth.
Again, your mileage may vary - but the general idea is that you start doing bald heads and simple clipper cuts (think all over with a #1 guard). This gets you comfortable with the routine of setup, proper caping, proper use of a Santek strip around the neck, chair setup, basic ergonomics, performing a simple cut while conversing with your customer, cleanup, customer service and post-haircut disinfection and sanitation. In many places you are allowed to keep guards on your clippers for the first 800-1000 hours, while you learn the basic mechanics of tapering hair and blending cuts to remove weight lines. Because you are not allowed to use guards on your clippers in the exam, typically you are expected to use a clipper over comb or scissor over comb technique for the last 500 hours. (Obviously, when you are licensed, you can revert to having guards, or even different size blades, on your tools.)
A barber school is a great place to get an inexpensive haircut. There is some degree of risk: and by having your hair cut there, you acknowledge that there is a nonzero chance that you will need to have a much shorter haircut than expected to "fix" something. But students are ALWAYS supervised by an instructor or master barber, and as a result should the student get hopelessly lost, there is a backstop to have a serviceable haircut happen as a result. As a student, where your college is determines the kind of clientele you'll get - if your school is in Chinatown, you will get a lot of experience with Asian hair. If it's in the black part of town, expect to do a lot of line-up jobs.
As you work, instructors will come over and adjust your technique, advise you to try another technique, assess your time management, fix a bad blend or overlooked reading of the hair growth pattern, praise you for a good job, and sometimes even take over if you're out of your depth. They also run interference if the customer gets irate, thinking that his $3 haircut done by a beginning student will rival a $25 cut at Vidal Sassoon. They'll also do a more thorough job of the first examination if need be: you may have only seen ringworm in a textbook, but they've seen it in real life, and will be able to advise you about what's a tumor, or a mole.
Barbering schools mostly make their money on client services, so you will be in essence an unpaid employee, doing the best job you can for nothing more than the occasional tip. But it's a fascinating way to do school - for someone who's used to a classroom followed by written exams and more school, having one day be with a textbook and the rest of the time involving actual tools and actual clients is a trip. Motivated students are already dreaming ahead: you'll see people trying out tools they've invented or are having patented, you'll see people trying different techniques they learned on DVDs or in books, and trading knowledge they've learned from each other. There's always been a camaraderie in a barbershop, and the college is no different.
Of course, you will get the occasional drug addict who falls asleep and then wakes up wondering when you're going to start (even though he has a temp fade he didn't walk in with) and the occasional backseat driver and screaming kid. But that's what the job ends up entailing, a richly rewarding (in experience, not money) profession dealing with all walks of life.
Just remember to log your hours, and look over at the other chairs every now and then.