In the past, I have read that lawyers simply 'read law' on their own or in an office, before passing the legal (bar) exam. But if they only indentured and never went to law school, then how did they learn jurisprudence or the academic theories of law?
In the U.S, at least for some time in our not too distant history, there were a substantial number of jurisdictions that allowed people to "read in" to a law degree, meaning exactly what @cpast said in his comment - that people who were so inclined and with the intellectual aptitude to understand old english common law and modern stare decisis (essentially, appellate precedent), as well as statuary and regulatory texts, could simply study their way to a law degree. It was assumed (quite rightly) that if one could manage to pass a 2-4 day bar examination that they should be considered eligible to apply for a license to practice through admission to the bar just as legitimately as those people who earned their juris doctorate. This is keeping in mind that passing the exam does/did not guarantee licensure/being sworn in, as in depth background checks and ethical fitness analysis are also conducted which form the final determination for fitness to be admitted.
California was, for a long while, known as the most challenging state's bar exam to pass. For a long while, it was a true "read in" state, meaning anyone had the right to take the bar exam. Because of this, the California Penal system actually turned out quite a few amazing street-smart criminal lawyers; men, who spent years in prison reading the law, who came out to take and pass the bar exam. It is also true that in CA, as well as other states, conviction of crime (even felonies) does not necessarily mean exclusion from licensure based on failure to pass the background/fitness inquiry (it can, but it isn't automatic if you can establish rehabilitation). Excluding crimes of moral turpitude, such as perjury, embezzlement, certain thefts where a fiduciary duty existed (those were the only crimes that were considered automatically exclusionary), one could be allowed to make a plea of rehabilitation to the Board of Bar Overseers ("BBO"). Crimes that account for the bulk of prison sentences, like aggravated violence, drug dealing/trafficking, etc. are not crimes of moral turpitude.
However, there are no longer any states in the U.S. where you can truly "read in" to the law. @Jason Aller is exactly right, that in the early 80's the ABA, as well as the association of accredited legal institutions (not to be robbed of their mortgage of the mind) lobbied and cajoled the practice right away. There are still the states that allow an education of apprenticeship, whereby rather than being self-taught, you are allowed to be mentored by a senior attorney with a certain level of experience, where they would attest that you worked a certain number of hours (usually each week or month – typically something akin to full time) under their tutelage. Each of the states Jason Aller cites has some form of this. After a certain number of years, those individuals are allowed to sit for the bar. However, that does not mean they can be lawyers ... even if they pass.
I recently read a journal article discussing how each year more and more states seem to further delimit who can be lawyers, for the sole purpose of not wanting to create too much competition in the profession. Background analysis now puts a great focus on financial responsibility: meaning if one's credit score isn't up to par they can be shut out of the practice because it's argued that if they cannot be financially prudent with their own finances, they cannot be trusted to manage client funds. About a quarter of all people who take the bar on the first shot fail. Those who don't pass by the 3rd try usually never do, and if they do, they are unlikely to practice as they are unlikely to get jobs (unless they hang out a shingle, so to speak). Most states require having graduated from an accredited law school. Massachusetts has at least one unaccredited law school (it was two, but I believe one just got accredited). A grad of that school can only be licensed in MA, nowhere else, except maybe one or two other states. More and more states are also deciding not to let lawyers from other states practice in their jurisdictions by "waiving in", which is where you can pay to get licensed and transfer your scores (instead of experienced lawyers having to take the bar again) from reciprocal states. It is a club where membership is purposely limited and continues to be more and more exclusive (and not in a good way). You can probably see from all these trends that there is a big push to keep people out, for no other reason than fear of competition. If this trend holds, it seems likely that the apprentice's right to "read in" will sooner, rather than later, become altogether a thing of the past too.
Law school education is undoubebly valuable in that it teaches you how to learn in a socratic way. It shows you how to spot issues and understand archaic text, and helps you to understand the rules of procedure in a theoretical and comprehensive way. However, for most, what it does not do is teach you how to practice law. When I was in law school, I was lucky enough to go to a school with the choice of a clinical semester or year (I also broke the rules and worked full time as a law clerk, year round, from year one). Many schools had no clinical programs. Students who weren't able to do these things didn't know what a pleading was or how to draft one, had never seen a motion for summary judgment, had no idea how to take or defend a deposition &ndash all things lawyers must be good at doing and that are not taught in law school. At that time, over a decade ago, the machination of legal scholars had contemplated adding yet another year to law school curriculum, to require a clinical year, so that graduates did have a clue what practicing law was all about. Opponents argued that instead the 3L elective year should just be traded from classroom to clinical (rather than adding another year and another 40-70 thousand tuition dollars). Neither side could agree so neither has become the norm. But what has happened since then is that more and more states have limited the right to take the bar exam by right, from having learned the law thru the practice of "reading in", which had already become the highly regulated practice of "documented apprenticeship". My guess would be that those persons who learned by apprenticeship may not be as adept at picking apart cases or legal research as a law school grad (but may be), but they would almost certainly be more informed in the actual practice of law.
Here is a fairly comprehensive article about the U.S. states that allow it, and what their rules are for completing the apprenticeship "degree". It cites that of nearly 90k people taking the bar, 60 took this route. From the perspective of a practicing lawyer with what I like to call "a mortgage of the mind", I find this trend pretty disheartening. One shouldn't need to pay a quarter-million dollars to gain the knowledge and right to sit for the bar.
In common law countries there was, and in some areas still is, the practice of "Reading the law" which was similar to an apprenticeship. A prospective lawyer would work for an established one until they were ready to practice on their own.
In the United States California, Maine, Vermont, Virginia, and Washington still have processes that permit this practice. The American Bar Association worked to limit access to this method starting in the 1890s.
The (ancient) Romans had schools of rhetoric, which was largely informal when compared to the current legal education system. Consul Tiberius Coruncanius gave legal classes in the 3rd century, but they would have been small.
Over one millennium later - more like 1500 years - universities in Europe began studies in law, mainly ancient. This wouldn't have been a huge problem, as the legal systems at the time were often based in part on the Roman system(s).) This formed the basis of the university legal education system still in use today. "Modern" law - the law of the day - would not be taught for several hundred more years.
The university system, in which law was concentrated on, developed in full about two to three hundred years ago.
Edward I, in 1292, made an effort to start a small legal education system to find people to staff the courts. The training was sometimes informal, but there was an underlying system, centered around the Inns of Court. These became the basis for the legal system in England, keeping their eminence into the 18th century. They became meeting places for judges and lawyers, not just students.
Oxford and Cambridge, not wanting to be outdone, started serious law programs at about this time, which led to the decline of the Inns of Court, and the start of the system I discussed earlier. the programs were at first informal, and not designed for pure legal training, but eventually became the center for legal education in England.
To add to the above, there is a growing gap between academic law and real law/practice of law. In the old days, people practiced law for a number of years, then they became law professors. The current law professor path is to go to Harvard or Yale, clerk for a federal judge, then go into teaching. A significant case can take a decade from start to finish. A law professor with even two years working at a law firm does not have real experience.
When I was in law school, there were two clear groups of professors. There were the generally older professors who had practiced and had real experience. There was a growing number of the idiot savants who had no real legal experience whatsoever but are now teaching. The first group is dying out and the second group is growing.
There is growing talk that there needs to be a change in legal education because of the growing gap between practice and law school.
And I am not just talking about the mechanics of law. Law school has never taught mechanics, such as all the paperwork you need to file when you make an appeal. Increasingly, it is not even teaching the actual law (when I go back to the school to listen to lectures by [new] faculty members, I leave scratching my head).
There is something to be said for the "reading the law" approach. As the disconnect between law school and law grows, this old very path to law will become more attractive and probably a better way to become a good lawyer.