Well, there's supposed to only be one true ending.
This was the plans, last I've heard:
"Harem Route
(You are here) | Dark Route | Purity Routes (Endings) | True End"
Personally though, I don't think Ami will be much of an enemy. As scary as the game tries to make her seem, Sensei can and has just sent her to her room:
I also struggle to see anything that is actually Ami, being a threat to Sensei:
He's more or less her reason for living and dying. If anything, based off Amy, Ami may just end up being a tragic victim in all of this.
Frankly, I'm more wary of Ayane, Maya, and Sensei, than I am Ami. She seems too obvious. Like whoever is actually a threat, wants you to be distracted by the creepy red head, instead.
Could definitely be wrong though.