Picture one of those Al Hirschfeld caricatures showing a scene from a Broadway play -- all swift lines and angularity and wit. Then imagine that caricature translated into prose. The result might well be "At Freddie's," a slim, elegant novel about a school for child actors in the center of London.

Freddie is the school's founder and its driving force, its very heart. A massive old woman swathed in shabby clothing and sprinkled here and there with semiprecious brooches, she rules her domain absolutely. Nothing commercial is allowed at the Temple School -- no TV or film work, no modeling. Her students are serious actors. If that means little money coming in, so be it. For Freddie wields the tyranny of the truly dedicated; the rich and famous are shamed over and over again into emptying their pockets for her cause. The Old Vic Theatre audience, at one point, sends her an offering of 400 pounds along with a note reading, "Shakespeare would have made it five." Noel Coward composes a song in her honor and plays it on the school's donated Bechstein grand piano, which, with two of its legs sinking through the rotting floorboards, gives the impression of "wading ashore." And Freddie -- rapacious, impervious, single-minded -- accepts it all as her due.

You might suppose that so formidable a figure would dim the book's other characters; but you'd be reckoning without the wiry strength of child actors. Balanced against Freddie's stateliness is her pupils' fierceness. They are streetwise and disturbingly shrewd, and they possess a manic energy that gives the book its sense of movement. "Feverishly competitive, like birds in a stubblefield, twitching looks over their shoulder to make sure they were still ahead, they all of them lied as fast as they could speak." They're a fascinating group to watch, and if you tire of observing them in a body you can always narrow in on the most intriguing one, the student acknowledged to be a genius:

"Jonathan . . . was silent for long periods, and was the only child at Freddie's who had no audition piece. He could no more be tempted into a display than a hibernating animal. Then, when he emerged, apparently knowing his own times and seasons, he would become something quite other, doing a speech or two, or dividing himself in order to turn into (for example) two elderly men he had seen through an office window, one short, one tall, getting ready to go home, and helping each other on with their coats. They dusted each other off, the short one stretched, the tall one discreetly bent down. All this was not so hard to imitate, but Jonathan suggested also their tenderness for each other's infirmities and a certain anxiety, about which he could have known nothing. After a bit the scene disappeared as he subsided, sticking his chewing gum back into his circular cheek."

As you can see from that passage, Penelope Fitzgerald is a deft and nimble writer. A past winner of England's Booker Prize, she displays the English gift for understatement. Her apt phrases are tossed off casually; her humor is flicked at us airily. Just watch Freddie with her brother, a staid solicitor:

" 'Well, it was good of you to come, James, and I'm interested you should have thought it worthwhile to do so. I think it will make you feel better. Why, this very evening, when you talk things over with your wife -- what is her name, by the way?'

" 'Cherry,' the solicitor replied.

" 'But that was your first wife's name.'

" 'I have only been married once, Frieda.' "

Or here's Freddie's assistant, Miss Blewett, comforting a teacher crushed by a failed love affair:

" 'Oh, Pierce, Pierce . . . you must believe what I'm going to tell you . . . even Sorrow has its uses. My fiance -- I always called him that -- died suddenly ten years ago, in 1952. Grief changed my appearance completely. And yet that led to the only position I've ever been offered at Harrods . . . Saturday salon model for white hair.' "

That airiness has its drawbacks, of course: It makes for a lightweight novel. Consciously, one senses, the author has chosen to stay with the surface of things. Even the story's turning point -- the moment when we realize that this is not a book about dedication after all but about power, pure and simple -- is flatly announced, and it's announced by a relative outsider who's watching from the wings, as it were. Our view of Freddie is always an exterior view. It's intriguing to the end, but never any deeper than what the most casual observer would see if he stood next to Freddie long enough.

Like a Hirschfeld caricature, "At Freddie's" aims solely to delineate, to depict. But it does that admirably well