While it would be best for Quinn to give a better show of his mana abilities while in front of students considering he was the left hand of the King, Leonor hated being fooled in such a way.
Considering she hadn't properly sparred since Stephano left Montclair, she hadn't been as aware of her surroundings as she should have been.
Quinn certainly knew how to read her. Even if he was moping before, she should have known he was sharp enough to push it to the side.
There was a massive difference between someone who lived their every second as a mage and someone who only used her mage abilities every so often. It was a wonder she wasn't worse off but her ability to have Eve awake and moving around even when she was asleep was what helped her still exercise her mana pool enough that it hadn't grown much smaller.