My vote is for Sleepaway Camp. When my friend told me about this movie, i thought she was making it up. She wasn't. It's about this camp where everyone gets killed and they are being killed by a girl who ends up having a penis at the end...
www.sleepawaycampmovies.com - yes, there is more than one sleepaway camp movie. In 2 & 3, the penis is gone but the killing continues.
It's a horrible movie, but if you watch it with some good friends who have good senses of humor, it is a GREAT laugh!