I yield to no man in my contempt for slut-shaming abstinence-only policies, but kids are not as malleable as supporters of comprehensive sex ed policies make them out to be. The available evidence suggests that abstinence-only programs have no impact whatsoever. Kids might as well spend the 40 minutes staring at a brick wall. It's a waste of class time and money, with some virgin/whore mythology thrown in for good measure.
You could argue that schools ought to convey accurate rather than inaccurate information about the subject. I would agree with you. But as far as I know, there is no solid evidence that "comprehensive" sex ed–the relevant alternative–has any impact on sexual behavior either. That's the conclusion of UC Berkeley sociologist Kristin Luker's extremely thorough book on the subject, where she explains why she can't find a single study robust enough to back:
We are looking for an outcome, teenage sexual behavior, that is affected by many forces, only one of which is sex education, during a period of tremendous social change, which has surely had some independent impact on such behavior, and we are looking at everything from one class room period to a semester's worth of classes, all in the service of trying to see if they affected the outcome.
Cross-country comparisons, for what they are worth, do not lend credence to the idea that kids take their sexual marching orders from public school teachers. In Sweden, kids are exposed to intensive comprehensive sex ed programs. In France, such programs are less common than they are here. Yet Sweden and France have similar rates of STDs and teen pregnancy. U.S. rates are higher.
As an aside, the most interesting thing about that study was not the jump in teen birth rates, but the jump in the overall birth rate. U.S. women are breeding above replacement for the first time since 1971.