"Girlschool Play Dirty Live." Sony. $29.95. These once extremely promising, self-billed "bad girls of rock" lost a charismatic frontwoman and distinctive edge when singer-guitarist Kelly Johnson left in 1984. She summarized the band's early appeal, which rested in its intriguing blend of the heavy-metal aggression of AC/DC and the good-natured lilt of the Go-Go's. Johnson projected a freshness that made the band seem like it was having fun with all the macho stereotypes of the heavy-metal world. Without her, Girlschool succumbs to those stereotypes and ends up simply plodding.