FMC has a lot of mana, but she wants to go to Knight school, and not the magical one.
So the only way for her to 'cheat' the magic measuring test, is to not get tested at all. (Ofcourse if her Father wasn't a high ranking noble there would be consequences, regardless of whether or not they can see why the thing was flung into the nearest wall, to avoid any further shitshow they just went with no magic detected.)
Cats gonna be cats^^ btw one thing always irks me in those villianess stories... why not be a good person?^^ if you dont act evil you wont have to suffer