The World of Henry Orient (1964) on Netflix

A mischievous, adventuresome fourteen-year-old girl and her best friend begin following an eccentric concert pianist around New York City after she develops a crush on him.

Henry Orient (Peter Sellers) is a madly egocentric and overly amorous avant-garde concert pianist who is hilariously pursued all around New York City by two fourteen-year-old fans. The girls, Valarie "Val" Campbell Boyd (Tippy Walker) and Marian "Gil" Gilbert (Merrie Spaeth) chase a harassed Henry all over the city, thwarting his afternoon liaisons with a married woman and leaving utter chaos behind them - until Val's sexually promiscuous mother, Isabel (Dame Angela Lansbury), appears on the scene to put a stop to the girls' shenanigans.

—alfiehitchie

After meeting as the new students at upscale private Norton's School for Girls in New York City, fourteen year olds Marian Gilbert and Valerie Boyd - Gil and Val - become the best of friends, especially in their mutual love of adventure and fantasy. Between the two, Val is the proverbial troublemaker, arguably due to having absentee parents, wealthy Frank and Isabel Boyd, who travel the world on his business leaving Val with paid caregivers, resulting in she never having had a proper home. As such, the Gilbert home consisting of Gil, her divorced mother Avis Gilbert, and their similarly divorced friend nicknamed Boothie, is as close to a home as Val has ever known. In their adventures around town, Gil and Val continually stumble upon who they learn is second-rate concert pianist Henry Orient - his own manager and childhood friend Sidney stating that Henry is no Van Cliburn - who likes to portray himself as "international" but is really a born and bred Brooklynite. A womanizer, Henry is pursuing an affair with suburbanite Stella Dunworthy, already skittish in her fear of being caught in that affair by her husband. When she sees him perform in concert, Val ends up falling in love with Henry, which leads to her and Gil's systematic stalking of him. Already paranoid by seeing the teenage girls time and time again even before the stalking begins, Henry and more so Stella become even more paranoid in believing they perhaps working on behalf of Stella's husband. What happens with Gil, Val, Henry and Stella is affected by the return to town of the Boyds, Frank and Isabel who individually have their own agendas with regard to Val and their own wants.

—Huggo