I would label the climax of the story to be just the narrator's encounter with Al's corpse in the putrid waters of Greasy Lake. All actions following this encounter contribute to the resolution of the plot; all actions before this are contributing to the tension of the story. The narrator's clubbing of Bobby over the head with the tire iron, the attempted rape of Bobby's female acquaintance, and appearance of Bobby's friends in the other car are all certainly peaks in the rising action, but it is the encounter with the dead body that leaves the reader most wondering what in the world will become of the protagonist.

There are several places in the story that you could argue are the actual climax.  In terms of the action, you could point to the entrance of the other car that scares the boys out of their plan to rape the girl they are with, you could also point to the narrator's run in with the actual body in the lake, a truly shocking moment perhaps more powerful than others in the story.  You might also look to the boys being labeled as "pretty bad characters" by the girls that show up, and their dismissal of the label as one they wouldn't like, though of course they would have loved to have been labeled as such just a few hours earlier!