I spent many years as a professor of law and dean of a law school. I think I know something about the place of a bar exam in the profession. It is not true, as your editorial says, that there is no "clear correlation between an applicant's score on the bar exam and his skill as a lawyer."
There are two professions (medicine is the other one) in which the practitioner holds his client's life in his hands--if not literally as in some criminal cases, but figuratively in making it possible for him to keep or acquire his legitimate property.
As you point out, the proportion of minority applicants who pass the California bar examination on their first attempt is small, but that is due to the fact that most of the minority candidates have prepared in poor law schools; you will find that the proportion of minority applicants who have gone to, and graduated from, the better law schools pass in the same proportion as do the graduates from those schools who are not of the minority groups.
Unfortunately, most of the minority young people who desire to become lawyers must, for financial reasons, work outside of law school and must attend the poorer schools because it is only at those schools that the economically poorer student can hope to acquire even a rudimentary legal education. Rather than object to the bar examination screening out those--unfortunately--poorly prepared candidates, you should use your influence to make it possible for scholarships or similar aid to enable these minority young people to attend full time at the schools that can properly prepare them to attempt the examination and pass it.