YOU CAN'T SAY YOU CAN'T PLAY by Vivian Gussin Paley (Harvard: $15.95; 134 pp.). While other kindergarten teachers might hear "You can't play!" as just another classroom noise, as natural as bawling and giggling, University of Chicago Laboratory School teacher Vivian Gussin Paley would hear portents of riots, poverty and other problems that arise when a ruling class of people gains the power to "notify others of their acceptability." Asking "Must it be so?" Paley makes this book's title her classroom's golden rule. At first, most of her kids are charmingly ambivalent. "They has to let her play," Sheila insists, "unless they really don't want to." But by the end of the school year, Paley does indeed notice gains. Lisa, for example, begins to befriend kids she had previously excluded: "Pretend we live in a castle, just you and me, Hiroko, in a beautiful castle of gold and diamonds." Lest all this sound too saintly sweet to be practicable, it should be pointed out that Paley has considerable ambivalence about her experiment, and her probing questions about its viability ("Is the primary purpose of play to have and to hold a best friend? Or to establish who's the boss?") should win a broad following for this inspired little book.