[cynicalicious]

Don't hate on Midori-- she's just trying to warn Ran, and she's acting out on her own past, as others have said.

Spoiler for The identity of the masked person:

Well, I watched episode two again, and it didn't take long to figure that one out-- it's gotta be the homeroom teacher.

1. She's new-- she said so herself. (So maybe she's a wandering villain type, or someone after revenge?)
2. The mask in the art room (kind of a dead giveaway there)
3. She's the one who stopped the fight between Ran and Midori-- Midori hinted at that. It wasn't Rui who stopped the fight, it was Momoko-sensei, who apparently knows how to use her powers better than Ran does.
4. Momoko-sensei knew what the fight was about (because she could hear Midori), so she didn't say/do anything. A normal teacher probably would have flipped out if one student just started yelling at the new kid and hit her. Momoko-sensei heard the whole exchange, but she didn't pretend not to hear it very well.
5. She knew to go after glasses girl and Rui, two people Ran knows.
6. One of those obvious anime rules, too