Vague memories from watching this in the 70s- a boy (main character) is around an old mine or gravel pit where other boys are in a club or gang-  I remember one boy zip lining, and then they surround the first boy (the main character) and then he might have to have an initiation of some sort-  but later the boy needs to find and kill a serpent or snake or eel in the lake (or filled up gravel pit?).  Later he finds it and gets pulled into a tube or cave or tunnel, but he makes it back out after spearing the snake (or eel) or something.