Looking for a children's book about a boy making a deal with a demonic piece of software

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 forboding 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 and 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. They boy 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.