SOLVED. Youth Sci-Fi / Fantasy / Mystery about a thought experiment, disappearing teacher, rescue in another dimension? [s]

[Caroline]

The year: 1984 or 1985, I was about 10 years old and read a book from the school library. It could be classified as some combination of sci-fi, fantasy, and mystery. I cannot remember the title or author. I think it was a red hardcover with the title above a cover picture in a white square. The story was more or less contemporary, and I believe the protagonists were either preteens or young teens.

The book: A teacher (science, possibly) tells his class that he has a theory that things only exist when someone is thinking about them. He wants the class to do an experiment in which they will put an item in the front of the room and all make a point *not* to think about it, and it should disappear. Unfortunately, most of the students don't like him, so they decide to not think about him instead. The teacher is so intent on the experiment that he isn't aware of his own existence at that moment, so with nobody at all thinking of him, he disappears. One student has a conscience and decides he must rescue the teacher. So the class must not think about him, and he ends up where the teacher is, and they must figure out how to get home.

Specific details I remember:

During the experiment, 'the teacher was thinking so hard about [whatever it was] that he forgot to think about himself, and that was his mistake.' Then he disappears.

The place where the teacher and student end up is a large void of some kind, I think.

Thanks for any help!

[Rainbowheart]

Mr. Radagast Makes an Unexpected Journey

[Caroline]

I think you've found it, Rainbowheart! Thank you so much!

[Rainbowheart]

You're welcome, glad to help out!

[Caroline]

I found it on Openlibrary.org and read it today. It's definitely the right book. :D