I have been having tremendous luck with story identification queries, so I figured I'd ask one more while I was at it. It was a pasteboard book cover, showed a set of foreboding gates — I want to say that it showed a realistic looking boy recoiling away from them — black iron in my memories. The story involved the protagonist at a boarding school learning that a computer program housed a demon. The demon offered him a wish, but said that, if his wish was not found worthy (I don't remember the criteria for worthiness assuming it wasn't just a matter of whether it amused the demon), he would arrange an accident for the boy. Said boy knew it was not an idle threat because the demon named a classmate who had excelled, but then had a tragic accident. The protagonist made a wish regarding his family, his parents I think, and then spent the rest of the day quailing at every sign of trouble. I vaguely remember him suddenly being aware of how many swamps and ponds there were around the school and I vividly remember him insisting on wearing a football helmet when playing outfield (baseball) at the school.
Beyond that, I have a vague feeling that either the boy's or the demon's name was in the title of the book, that it was short, maybe 20-30 pages, and that I read it somewhere between 1986 and 1993. Oh, and the boy got his wish and was not killed, but I don't remember what happened after that. Part of me wants to say that the computer the program was found on the computer of the headmaster, implying that the headmaster was making judicious use of the demon to shore himself up. And there was something about a prior wish-maker being an opera singer who wanted to be able to sing the high C and succeeded shortly after her fatal accident caused her scream to hit the right pitch (I'm not certain on the accident, but I know there was an opera singer).
I did look through the bookshelves at my parent's house the last time I was home, but to no avail. Apparently, my mother cleared out a large number of books and this may have been one of casualties (it probably wasn't in the best shape, frankly. Our books were well-used, as typical with kids of that era).