I'm not convinced by the "it's fake/delusions" theory.
Aside from the heroine's narration (and her name) the story also flat out labels her as vampire when introduced. The bite in ch.1 looks real, with blood around her mouth, and in ch.2 you see the bite marks. The "victim" collapses, and was attacked from behind - how on earth do you "delude" yourself into that having happened that way, and having "your victim" then go along with it spontaneously? Nothing in ch.2 or 3 points specifically to any inconsistency with any of those; the vampire features are always present, there's nobody in the background or anything that does anything contradictory, the classmates don't react like it's unreal or anything.
Lastly, and to me most importantly, the trope of having changed pupils is always consistently there too. And we know playing around with character's pupils is serious business ;)
If this is supposed to culminate in a "plot twist" then I gotta say from what's shown so far it's not well done. There should at least be some reasons for doubt, but there's nothing.
What I could imagine is some sort of rule like enthralling only works if the person goes along with it willingly or somesuch thing. But otherwise this doesn't really check out for me ^^#