Every study carried out on the topic has indicated that this is untrue. Schools which teach contraceptive methods, and even provide students with free condoms, do not see their students having sex from an earlier age. Nor do they have sex more often. This is a myth with no empirical evidence to support the claim. 
On the other hand, there are many studies that show the benefits of teaching children about contraceptives in schools. In 2007, a study published in the American Journal of Public Health found that teens that use condoms the first time they have intercourse are half as likely to contract chlamydia and gonorrhoea over the course of the following seven years. This shows that those that start off using contraceptives, stay safer for longer than those that don’t. 
The argument that sex education normalises sex and therefore shouldn't be taught in schools is also flawed. Sex is already normalised at a young age. Teens are exposed to it every day on TV, in video games, on social media and in their friendship circles. Teaching it in school isn’t going to have any impact on promiscuity, it is simply going to ensure that when teenagers start having sex, they know how to protect themselves.