I saw this TV-series about a High School in the US. One of those schools that teach abstinence only. I think that is stupid! Very much so even.
If you want your kids to not get pregnant and not get any STDs the best way to avoid this is NOT through teaching them that sex is baaaaad and that they shan’t do it.
Best way to avoid teenage pregnancies is through information. Lot’s of it and at a fairly early age.
Teenagers will have sex even if they are told not to. That’s just the way the world works. So to make sure that they have safe sex they need to know what safe sex is.
I read on this site that :
“The United States has the highest rates of teen pregnancy and births in the western industrialized world.”
I also read that :
“The good news is that teen births have dropped by almost a third since the beginning of the 1990s. With pregnancy prevention programs and more understanding and teaching about safe sex, this number will hopefully drop even more.”

So the best way to stop kids/teens from doing things that can or will harm them (like unsafe sex or drugs) is information, lot’s of information!

During the ’70s in Sweden there was a LOT of information about drug use and safe sex, so in the late 70s and 80s drug abuse, STDs and unwanted pregnacies dropped, only to rise again in the 90s when the amount of information easily acesible dropped. Now they’ve again increased the information available and as a result the teenage pregnancies and abortions have dropped again.

So basically the most common ways to avoid pregnacies would be condoms, “the pill” (what is the proper name for that anyway?) or not having sex.
Not having sex is the safest. It works in 100% of the cases.
Condoms are safe in 98% of the cases, and pills in 99.7-99.5% of the cases depending on which kind of pill.
(Numbers from wikipedia, if they are wrong it’s not my fault xD)

So in a way abstinence is the best birth control methodm, but it’s also the most boring. If there’s no pressure and you’re after the same thing, sex is great. And people who are ready for sex are not always ready to have kids.