by Angela Davis-Gardner.Random House, $15.50. The time is 1921 and the place is a French Catholic girls’ school in Nova Scotia, where the pubescent Felice is a bright but difficult student and the idolized Sister Agatha is going outrageously dotty. Ms. DavisGardner is adroit in conveying the mixture of juvenile mischief, romanticized piety, and suppressed sexuality that pervades L’Académie du Sacré Sang. It is not the least of her achievements that the reader, offered the story through the limited vision of Felice, comes to feel deep sympathy for the Mother Superior who has to manage the place.