What about this study?
articles.cnn.com
EXCERPT
An abstinence-only education program is more effective than other initiatives at keeping sixth- and seventh-graders from having sex within a two-year period, according to a study described by some as a landmark.

The study, published in the current issue of the Archives of Pediatrics & Adolescent Medicine, indicated that about one-third of the preteens and their young teen classmates who received an eight-hour abstinence lesson had sexual intercourse within two years of the class.

By comparison, more than half of the students who were taught about safe sex and condom use reported having intercourse by the two-year mark, and more than 40 percent of students who received either an eight- or 12-hour lesson incorporating both abstinence education and safe sex reported having sex at two years.