IS THIS A CHILDREN'S book, a teenage novel? Not, one would have thought, unless Romeo and Juliet is a teenage play. True, it is about adolescene - like Catcher in the Rye with comedy the dominant tone instead of angst. Catcher in the Rye was published on the adult list, but then that's some time ago. The way the world is going, adults stand to lose a lot of fun, for they would surely appreciate Jane Gardam's special quality as readily and thoroughly as young readers.

Marigold Green is a housemaster's daughter. Her mother is dead, and she lives alone with her father, Polly the housekeeper, and dozens of boys - it is a British boy's boarding school. Her father is Bill, and schoolboy humor ensures that Bill's daughter becomes, and stays "Bilgewater." Polly is loving, but has strange ideas about dress, and Bilgewater grows up feeling, and looking, a fright. Her social life is either non existent or disastrous.

The Headmaster's beautiful daughter takes Bilgie in hand, gets her hair cut, and buys her some clothes, but by and by gets bored with her. Bilgie adores Jack Rose, the handsomest boy in the school, (though his eyes are too small); when he shows in his true colors as a dreadful cad, she ricochets into the arms of Terrapin, with whom she has lived all her life - he didn't go home in the holidays - without knowing him at all. And this delectable first love goes wrong at once, as first loves will, without one exactly knowing who failed whom. All the while the Cambridge entrance exams loom, and Bilgewater's vague, scholarly father is getting his life and hers into more and more of a mess.

Jane Gardam is a humorist, and a stylist. The language - dry, clear and sparkling - would belie the first-person voice of the callow heroine, were it not that though naive, she is brilliant, the sort of person who really might, in later life befome . . . but one must not give the plot away. A part of the humor lies in choosing very eccentric and extraordinary people to write about. Really there can be few girls so odd, and oddly circumstanced, as Bilgewater; but a transformation is worked on all this oddity by Jane Gardam's clear daylight, vision. Bilgewater does not seem odd, but totally understandable and lovable; instead we perceive that anybody's life is starange and extraordinary, that life itself is crazily particular, arbitrary, full of comic accident and peculiarity.

Will this kind of social comedy survive an Atlantic crossing? That's hard for a British reviewer to judge. The United States, as far as I have heard, has no schools like St. Wilfred's. So here's a test: Bilgewater goes to stay with the parents of the glamourous Jack Rose believing them to be doctors, but when she gets there she finds they are dentists; her disillusionment has begun! British snobberies can bery very delicate and perplexing; much of Jane Gardam's sharp bright comedy depends on distinctions as fine as this.

Delightful as it is, this book is not faultless. The plot does run rather wild in places, and though many a girl has like Bilgewater gone off with neither of the two she was involved with, but with a hitherto unregarded third party, the reader might have liked to know more about him earlier, might like to know what he looked like. It's disconcerting when a well-loved heroine departs with a shadowy figure, leaving two brilliantly envisaged characters behind her.