RiderFan said: Blunt force trauma rarely causes lacerations. You'd have to smash your skull into shards and have those pierce your skin.
At the risk of applying physics to a rifyu comic, I'm guessing it's an abrasion of the forehead from moving her head so quickly that it scraped lengthwise across the desk.
As with scalp wounds, even a small wound of this sort would probably bleed profusely, as seen.
Owwwww, my head hurts!I'm taking her to the nurse's office!Oh, you, Sayaka!My head hurts, may I go to the nurse's office?I pretended to be sick so we'd be alone together.