Natural family planning is where you have a calendar on the wall, and you write down exactly when the woman gets her periods, and you try to predict when it will be "safe" to have sex, and when there is a high risk. You then just avoid having sex during a high risk time. I think you can get very complicated kits which are actually quite scientific and you have to take the woman's temperature every day or something, and you can plot graphs showing the "safe" times and the "unsafe times".
It's basically what Catholics use, because they don't believe in any form of hormonal or barrier contraception, on the grounds that if God wants to give them a baby, it would be a sin to refuse, or something.
But the "safe period" method is why Catholic families have so many children. It's not safe, as every woman's cycle is different, and the only way of finding out exactly when your "safe" times are is trial and error. Which means sometimes there will be errors.
It's all very well doing this if you're devoutly religious, stay a virgin until you get married, and then practice natural family planning with your spouse. In that situation it isn't the end of the world if you do get pregnant. At least you're married and stable.
But I think it's highly irresponsible to be promoting that to teenagers who have no intention of remaining a virgin until they get married, and have probably already had sex anyway.
Just a final thought - if it's all the chavs who tend to be the ones stupid enough to get pregnant accidentally, surely they're diluting the average intelligence of the population?