Yup - my latest save was on a friday, so I had to skip the whole weekend and skip classes on monday too so that her event triggers on a tuesday, when there aren't two classes, so that I could hand in the assignment in the evening, not at night. Still, this is clearly a bug in the game's coding - if you can't do the event on days with 2 classes, it should just not trigger on those days... Anyway, thank you both for your answers and have a great day!