Answer: Twins, Sam and Eric are on fire duty when they see the dead corpse of the pilot, still attached to his parachute. Positive it is the beast, they run back to warn the other boys. The bigger boys head off in search of the beast. On their way, they discover Castle Rock.
I believe this is might be the answer. I actually read this book, and chapter 6-7 scared me due to this sole reason.
Though I am not sure if your teacher wants a specific reason from the book that you guys may have went over in class.