A gem. Director Amy Heckerling, whose FAST TIMES AT RIDGEMONT HIGH (1982) defined a generation of suburban proto-slackers, goes back to high school -- this time in Beverly Hills -- and the result is pricelessly fresh and funny. Cher (Alicia Silverstone, who's perfection itself) and her best friend Dionne (Stacey Dash) are both fabulously rich and, well, more