She desperate for the D, and when I say D, I mean the D and Divine Sword. Remember cultured people, Chikyuu logic don't apply to isekai logic. It's green lit bois! ( ° ͜ʖ°)
The sword spend a day brainwashing her into the joys of MCs D.
Also, their duels are dubious. They have to fight the current students and if they loose their out ?
Isn't this just a blatant lie? All the plot development so far has hinged on his lack of confidence.
If he had confidence, the story would be something entirely different.