Has everyone already forgotten how he fought with the principal?
That was taken place in a human-centric academy, though, and it was from Ephthal demonstrating his magic that Merlin took him and Anastasia to the academy for demons.
That time, nobody there knew who Ephthal was, and there's the stigma that humans are incompetent in magic. The only time Ephthal somewhat demonstrated his capabilities was his spar with Anastasia's former master of her slave brand, and some people who were watching in may have called it a fluke, or a stroke of luck. Maria, however, seems to view Ephthal as competent enough to at least follow the other students' trail from that one spar alone, not even having the chance to witness his real magic yet. Hence why she's offering them to join her in such a condescending manner.
Why haven't the students started looking at Ephthal warily after he destroyed that former master's estate, got him expelled from the academy, and exposed his family of all the illegal activities they have pulled over the last few centuries? My guess is that Ephthal asked Merlin to keep his name from being mentioned when it was reported. So as of now, the students are aware of that former master's evil deeds, but they've yet to know who pulled it all off. Ephthal is still known as the weird human who was able to make that cheap shot at that former master in the past, and his true capabilities is still being questioned.
This is only my speculation, however. I didn't see any hint of this ever mentioned in that aftermath.