First off the history of the doctor that evaulated the MC.
She was sent off the war at the age of 10 for having a S ranked power. Yes she was sent off as a child.
She survived the war because she was saved by the S rank Time controller, which later adopted her, because the country wanted to cull her due to how young she was and how powerful she was.
She became a doctor / evaluator so she could in a sense protect young kids like her who have no choice.
Now the MC.
When the MC went to visit her, she was mortified because she knew exactly how OP he was.
Yes the spoilers failed to note this.
She knew it was on the simular level of her foster father who could control time.
These users are called [special[ principle users.
To make things worse, the MC wanted to go off to war, fight and become a hero.
Remember the Doctor has been there, to her these "Heros" are monsters who slaughter and destroy landscapes.
So she was terrified, that if one day he became a S ranked, he would be deemed a threat on {Disaster} level.
But right now the MC is not there yet, and she litterally wants to crush the sprouting bud so to speak, so she tells him that he is trash.
She reports to the country that he is a trash, and decides she will watch over him personally, so he isnt subjected to the same stuff she was. (thrown into war at a young age)
She resolved to accept all the hate it may bring, if it will protect him and the world.
This is why you see someone this high ranked at a F ranked school.
She is here specifically to watch over him, and make sure he doesn't do something S ranked which will get noticed later on.