Ami-sensei is Cure Dusk.Think about it...
- Two Moon Pieces have been in her possession so far (the paperweight and the communicator), and a third is said to be inside the school (according to Mireyes, who, in contrast with some villains we know, seems to be a reliable source). Unlike Star Pieces, Moon Pieces don't just gather in clumps like that. Sure, Asa and Yoko suspect Dawn of planting the Moon Piece in the classroom, but if Ami's telling the truth about getting the paperweight in the shopping district years ago, Dawn couldn't have planted it, and if she's not telling the truth, why is she lying? Because she knows more than she lets on, that's why.
- She believed Hoshi, Yukari and Mia when they gave a weak cover story for Asa, Yoko and Emiru being missing from school. Secret Keeper behaviour right there.
- Dawn hangs around the school after hours in episodes 2 and 15, knew Yukari's name in episode 6, and just always seems to show up when she's needed, even though, as she says, "Do you think I have nothing better to do than to stalk a couple of schoolgirls?". Okay, knowing all that stuff is easy — it's a small town. Still, it might point to some relation with Ami, who would have a reason to know things about Clair Academy.
- Look at the important classmates' names. Ogata Mia, named after Ogata Megumi and Ikumi Mia. Okamoto Yukari, named after Okamoto Keiko. Kondou Hoshi, named after Nana Mizuki. Mitsuishi Seira, named after Mitsuishi Kotono. If they're all Named After Somebody Famous (except Emiru, who's the Odd Name Out due to becoming Millusion), then why isn't there a famous Nakata who's a mangaka or voice actor from a magical girl series? Answer: look at Yoko's and Asa's last names. Nakayama + Kawada (the "da" can be read as "ta"). The wiki page even uses the exact same kanji. Obviously, something's important with Ami-sensei.
- She built the communicator and made it work. Sunbi and Moonla must have talked to her if they knew about the communicator and didn't ignore Asa and Yoko, who they'd never seen before. Do you really think she thought she was contacting Mars?
- Justice speeches. Unlimited justice speeches.