A few morsels to tempt you to read the whole thing:
In the early '90s, the Dallas Times Herald hired a new theater critic who, it turned out, had been a professional actor. He even still had his Equity card; he'd acted off-Broadway.
Then one evening I ran into a theater director/professor/friend and happened to ask him what he thought of the new critic in town. He was curious to know more about the fellow's background, and when I explained, my friend promptly quipped, "Oh, yes, that's what every director wants judging him from the seats. An ex-actor with a grudge."
Well put. What this says is that not only is direct education/experience in an art not absolutely necessary for a critic, it can even be a hindrance, a distortion. Your teacher could have been an eccentric, someone with a warped notion of his art. Or, considering the highly collaborative nature of an art like theater, extensive background as a lighting designer may cause one to over-value the importance of blue gels and undervalue the contributions of, oh, say, actors.
It's with the word "earn" that I realized how this all actually works. We have it backwards. The critic doesn't bring authority to his reviews. It's his reviews that grant him authority, earn him any authority. A review is not an opinion, as Mr. Schickel says. It's not even (just) a wise judgment. It's an attempt at persuasion. It doesn't simply tell us that this book is worthy of our time and attention. A review tells us why and how. In trying to explain the critic's own response, the review justifies them. The review leads us into sharing his conclusion.
In other words, the critic earns his authority by using his knowledge, his rhetorical skills, his humor, his personal insights, maturity, modesty, bravura cleverness -- whatever it takes, in this particular instance, to convey the experience of the film in question and to convince us not only that he's right but that he's worth listening to. These are the only things that matter with a critic. Just as with a teacher, it's all about the classroom (and how he handles the homework), with a critic, it's all about what's on the page. If he can't do that, all the rest is meaningless.
He has lots more good stuff to say.