I'll Make a Habit of it(1994)

Idol Shiratori Nagisa misses combat and martial arts, so she dresses up as a schoolboy and enrolls in the most roughed-up school she can find. She develops a crush on one guy, who...thinks she's a boy... But it works out. Now she has to juggle life as an Idol and a schoolboy. All the while, wannabe Idols try to steal Nagisa's spotlight. And in Nagisa's wild world, there's nothing that can't happen.