Hilarious tale of life as a high school senior in the '50sVirgins, by Caryl Rivers. New York: St. Martin's/Marek. 246 pp. $12.95

''When I think of us, as we were then,'' muses Peg Morrison, looking back at herself and her best friend as high school seniors in 1956, ''I marvel at how vulnerable we were, and how brave; how little we knew of the world out there that would do its best to crush our spirits, break our dreams as if they were walnuts, herd us back into the tiny pens that were the part of the world that belonged to women. But for the moment we were young, we were free, and we had each other.''

This first novel is a hilarious account of that senior year at Immaculate Heart High School in Crystal Springs, Md. But it also includes tragedies, so it is both a laugh-out-loud and cry-out-loud tale of growing up.

Peg and her best friend, Con, a sophisticate from Long Island, are, respectively, the managing editor and the editor-in-chief of the newspaper at this girls' school. They have more ambitious goals than the other girls: Peg wants to be a Pulitzer Prize-winning reporter, while Con intends to write for The New Yorker. At the moment, however, their immediate goal is, as Con says, ''to be the Messenger staff that will be remembered as long as a single brick of Immaculate Heart High School is left standing under the sun.''

And so, to ensure their immortality, they embark on such capers as inventing a saint (St. Leon of Skorytt, a.k.a. Leon Trotsky) for the newspaper's ''Saints Corner'' and smuggling a boy into ''the assembly on Christian Marriage, known to the student body as the Big Sex Talk.''

The boy involved in this adventure is Sean, Peg's boyfriend, a senior at Sacred Heart, the boys' high school. Peg has known Sean since she was 2; for her , friendship has grown into love, but since he plans to become a priest, her future with him doesn't look too bright.

Con convinces Peg that she should therefore date other boys, and in one of the funniest chapters the girls go to Annapolis for the Winter Hop. As the evening progresses, horrified Peg feels ''as though I had just been air-dropped into Sodom and Gomorrah.'' She returns with relief to Sean, trying not to think of a future without him: ''I felt like a woman in wartime, pretending that only the present was real, that tomorrow simply didn't exist.''

Anachronisms are a risk in such a book as this. Ms. Rivers usually keeps the up-to-date slang in the narrative, not in the '50s conversations, but there are a few slip-ups; for instance, we didn't say, ''Way to go!'' in 1956, at least not in my neck of the woods.

Yet the main thing that distracts from the delights of this novel is the surprising number of printer's errors. More of these do seem to be cropping up these days.

Nevertheless, this fast-paced, sassy novel triumphs, managing to be reflective and serious through all the laughs.