Unable to find Jill, Bobby couldn't concentrate during class. And the fact she didn't answer any of his message didn't help.
But while Bobby hadn't seen the note Jill had left, somehow his mother did. Said mother didn't know why, but she believed part of it. The part about Jill being a succubus, to be precise. The rest, she thought it was part of some dubious plan to take his soul, maybe a way to make it more challenging…
And the face Bobby made after she made him read the message, once he came back from school, seemed to prove it.
What the mother hadn't anticipated when she claimed she forbade Bobby from trying to see Jill ever again, was that since he had been touched by a demon, his despair and rage would consume his soul, changing him into Bohrrom the Incubus.