When Melanie Martinez released her debut album, Cry Baby, in 2015, she created a vulnerable persona in the titular character that connected with a large audience. For her follow-up, K-12, out Sept. 6 on Atlantic Records, the alt-pop artist chronicles the stress of growing from a toddler to a teenager. K-12 quickly evolved from a concept album into a full-length feature film, for which Martinez started writing a script in 2017. In the 90-minute movie (which Martinez, 24, directed), Cry Baby, a sensitive girl with magical powers, is sent to a disturbing sleep-away school, where she battles everything from mind control to racial and gender discrimination. “My main goal was to display school as a condensed version of life,” says Martinez of the film, which will be shown in…