After spending almost a full two days going through an internet database of anthologies and searching each story title that sounded remotely familiar, I found it! I can't believe it...

The book is "Superhorror" edited by Ramsey Campbell from 1976 (sometimes listed as 1977). The story is "The Viaduct" by Brian Lumley. It's actually two boys, though, not a group. Very intense story. I found a few copies of the book and have ordered one. It does have a spooky tree on the cover. I also found the story in a collection of Brian Lumley's from 1996 called "Fruiting Bodies and Other Fungi."

Thanks for the suggestion of the other story. I searched for it right away, and I can't wait to read it.