I agree the terms need work, and to agree with you, films that use the "what-if?" construction are not by default fantasy. What if the Olsen Twins went to New York? What if Carrot Top owned a supermarket? What if a guy as old as Michael Douglas could seduce and marry Catherine Zeta Jones? What if naked women lie in wait for the pimply pizza delivery guy? (Okay, those last two are sorta fantasies, but not the kind we're talking about.) There needs to be something more, and I think that's where arguments both for and against break down.

Using Jaws as an example is okay, because for the purposes of this discussion, we're including horror that contains a supernatural component under the heading of "fantastic". The shark in Jaws isn't just territorial, it's got a bone to pick. (Admittedly this is more true in the sequels, but even in Benchley's book there's the suggestion the shark is more than just a shark; it's almost a form of divine punishment.)

Moving back to Brick, it isn't the lack of adults that makes it a fantasy; it's that the characters are adult noir archetypes overlaid on their high school counterparts. I strongly doubt the writer or the director considered Brendan, Laura, the Pin, or Emily to be realistic portrayals of modern high school students; they were using high school as a fantastic setting for a much older and darker story. If it's not fantasy, it's close.

Of course, checking IMDB for character names, I've discovered that Brick was a last minute 2005 release, so it's out of contention for the purposes of this panel in any case. Bah.