Since she's a sixth year, she'd no doubt know Hit, Freyja, Cronis and Evander.
Cronis is her housemate too, so there's that. I imagine they should have some kind of preestablished something, though I don't know if it would be a friendship or what.
Is you want and enemy/rival for her, Hit's usually pretty good at getting on people's nerves just because of the way she is. Plus, judging from her app, if Mia is childish and sarcastic, Hit wouldn't take to her too well.
Apparently, Freyja tends to not have problems with people (aside from Pavo at some point) so I think they could possibly be friends.
And then I'm drawing a blank with Evander because he's being fickle but that could be just as fun if we throw them into a thread together.
On the other hand, I have Aleksa who's a house mate as well but she's younger. She's a fourth year. But she's kind of like Mia lol.
Then Jonathan Finch-Fletchley who's a third year Hufflepuff. He's afraid of pretty much everyone and everything for one reason or another, so he could be someone to be picked on if Mia goes that way.
And finally there's Kaiden if you ever have any need for him. Just throwing him out there since I don't get to use him often.