Invisible Girl (Out of Sight, Out of Mind)

Summary from buffy.com

     As the Spring Fling dance nears, Cordelia's new boyfriend is mysteriously beaten
            up at school -- by a baseball bat with no one attached to it. The attacker leaves a
            cryptic message on the locker, which leads Buffy to believe it wasn't an ordinary
            monster. Giles suggests it might be an invisible creature or a ghost.

            Another attack finds Cordelia's friend Harmony flying backwards down a flight of
            stares, with no apparent outside force. After hearing a girl's voice, Buffy pursues
            the invisible attacker to no avail. Buffy enlists Willow's help in trying to get a list of
            missing kids -- perhaps this invisible girl is one of them. Meanwhile, Angel appears
            at the library and says he's worried about Buffy -- he knows that something big is
            about to happen but he doesn't know what.

            Shock of shocks -- Cordelia is elected the May Queen for the school dance. Buffy
            is convinced Cordelia is the link between the two mysterious attacks. She finds a
            girl in Willow's printout, Marcie Ross, who she thinks might be the one. Marcie
            was in the band, and the invisible girl keeps hiding in the ceiling above the band
            room. Buffy follows her there to find a nest of sorts where the invisible girls has
            been hiding. Another attack occurs: Cordelia's teacher is almost suffocated.

            Further investigation reveals that Marcie was in everyone's classes, but no one
            seems to even remember that she existed. Giles says she simply became
            invisible because she was perceived that way -- it's all due to physics, helped
            along by living above a Hellmouth. Marcie's goal seems to be to destroy Cordelia,
            who is already wigged out by the attacks on people close to her. Cordelia's
            coronation as May Queen that evening would be the perfect place for Marcie to
            attack, but Cordelia is none too thrilled about being used as bait. As Cordelia gets
            ready at school for the dance, Marcie yanks her up through the ceiling. Meanwhile,
            Giles, Willow and Xander hear some haunting music and try to follow it, hoping to
            find Marcie. The trio is led to the boiler room where they are overcome by gas
            fumes. Marcie anesthetizes both Buffy and Cordelia, brings them to the Bronze
            and ties them up. Marcie, armed with an arsenal of creepy surgical instruments,
            explains to Cordelia that she intends to give her a face no one will ever forget. She
            slices Cordelia's cheek, but Buffy manages to free herself and a scuffle ensues.
            Buffy finally knocks Marcie out, just as a pair of FBI agents arrive to take Marcie
            away. Angel saves the unconscious gang in the boiler room, but Giles tells Buffy it
            was a janitor -- he doesn't want to remind Buffy of her lost love. Cordelia acts
            grateful to the gang, but becomes her normal obnoxious self when her
            newly-healed boyfriend and her cooler-than-thou friends show up.