The MC has a crush on the heroine but discovers she is a vampire. Now that he knows her secret he helps her in trying to hide her identity to others. I remember MC has a childhood friend that is either in the newspaper club or just loves taking pictures. The principal or headmaster is a demon that likes pranks. I remember this was a weekly manga and it is completed. I also remember it has an anime but for some reason i can't remember the title xD
Jitsu wa Watashi wa? I haven't read this yet so I don't know if it has everything you are looking for.
It's the english title of Jitsu wa watashi, so technically you've alreadu seen it Actually, I am... is the literal translation of the title while My monster secret is the official translation of the title