Wish Upon the Pleiades

One day, while visiting her school's observatory in preparation for a meteor shower, a girl named Subaru comes across a mysterious greenhouse where she meets a frail boy. After leaving the room, finding it revert to the observatory before her eyes, Subaru encounters a small alien-like creature and follows it to a magically sealed classroom, where she comes across a group of girls in witch outfits, including her childhood friend Aoi. The girls and the creature, revealed to be a Pleiadian and the girls' president, notice potential in Subaru and give her a special broom, which brings her up through the clouds to a star-shaped gem that transforms her into a witch herself.