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 ^^#
Eh, either way, the writing is inconsistent
If she's a real vampire, it doesn't make sense how she's been subsisting on tomato juice, never drank blood before in 400 years, is still a massive virgin, and chooses to attend high school somehow despite the expense of faking documents
If she's a chuuni, most of this makes sense as she is established to have trouble making friends and may have invented a fantasy about herself as a result. However, you have to deal with the eyes, the narration, and the blood drinking scene at the beginning all being somehow a red herring based on her fantasies
Personally, I think that the author just wanted the story to be a certain way and wasn't too concerned about explaining why it was that way. It will still be a fun read no matter which way it goes