The war is over, the cycle of school years remains. One of these years she's going to retire, but it just sounds so tiresome having nothing to do.
This year has another overeager third-year who wants to take every elective, which means setting up a Time-Turner, which means casting several diagnostic spells to check it for manufacturing defects before layering it in protective enchantments and keying it to the wards.
This one, it transpires, has a very serious manufacturing defect.