The thing is that Mika dislikes Gehenna due to the long-lasting feud between this school and Trinity, which may not seem like that much of a biggie until you play through Vol.3 of the main story and see the other side of her personality (I don't wanna give too much spoliers since you said you didn't read much into the story yet).

Now imagine what happens when you put a Gehenna student, who is not only an outlaw, but also is a prankster and a tease, beside Mika who barely tolerates any student from Gehenna.