Bought this around 1986 (6th grade book fair!) and cannot remember the title or main character names. Loved the story though, and hope I can find it for my daughter.
The hardback cover was green, and the picture was of three girls singing and stirring a fourth girl in a pot.
The story was of a girl in her early teens at summer camp. She ends up feeling saddled with another girl who is strange and a little disgusting – the weird girl borrows a barrette from the main character and uses it to clean her toenails.
There is a talent show (depicted in the cover art), and the weird girl is the one in the pot.
The main character ends up tossing the weird girl out of a canoe, and leaves her bobbing in the lake. She tries to make up with her before they leave camp, but the weird girl yells at her in the cafeteria.
The main character goes home regretting what she did, and tries to think of some way to make amends.