Most don't. I'm not surprised as it seems a no brainer right? To tell kids age-appropriate information so they can prepare for certain life events in advance and handle them better. There's been a time for all of us looking back when we would think 'I wish I had known this or that before it happened'. To know about changes in puberty before hair sprouts out seemingly everywhere, to know how to ask the person you like out on a date, what to do with a used condom without making it awkward.
Yet, that doesn't occur in every school. Yes it's much better than the old days but schools in England and Wales are only encouraged to have sex education lessons as part of the PSHE programme and to include the biology and anatomy of sex and STIs including HIV- but this only applies to some not all schools. Scotland just has to make up what they see fit for the students and there is no compulsory element to the lessons at all. As for Northern Ireland, they are limited even more so as to what they can teach, with many schools opting out.
Huge subjects like: what is consent to sex?; what is acceptable in touching of other people?; knowing as a male what a period is so it's not seen as weird; where to go for certain types of sexual health testing; how to talk to a partner effectively about sex and pleasure (the aim of the game); what different sexualities are; what transgender/intersex/non-binary gender is; the pressures of media and film in forming what is normal for our body; and how to be a good parent.
So what are they left with? Well porn mostly. This isn't all bad and many could say it did them no harm. Which is true as it can show some types of sex and at least what human anatomy (although not exactly of average sizes) looks like. The problem with porn is that too many young people just don't know that porn is actually just for entertainment. It's not a documentary and educational device but something to keep you company when you're single, or to spice up a relationship, or just bored. As useful as porn is, for some kids, and increasingly more, it imprints a lot of stereotypes that sex is all about the man's pleasure service, that all sex ends with ejaculate on a girls face, that penetrative sex will always get a woman howling with glee, all girls should swallow cum, everyone does anal, girls who have sex are to be called sluts and a Uni house party isn't complete without someone having sex in the middle of the lounge with a crowd around to provide moral support. Most reading this will either laugh or not see it as an issue, but a young person's mind is more malleable and likely to take seeing these images as the norm for sex.
Porn isn't the only contributor; far from it. Friends and their stories and crazy tales they have heard are also a big player. 'Chinese whispers' (maybe not the PC term now) can often make the story turn into quite a false legend of extraordinary folklore. This is always going to be a part of life and without it could make things boringly factual too much of the time, but kids often don't have many reliable places to go for information. If the kid doesn't have internet (well how could you read this?) as many kids certainly don't, where can they go? School is a certain place for information to be expressed and explained to prepare a child for their life ahead. By not having sex education we are missing that opportunity.
But what about the parents? There should undoubtedly be a partnership between schools and home, but if that adult hasn't had sex education themselves then how are they to know exactly how long an implant works for; what STI symptoms are for each disease; the anatomy of the uterus when explaining about birth; and how to help a child questioning their gender. It also makes it harder to talk about these subjects in a culture where sex is the Voldemort and 'that which must not be named'.
Efforts are being made to change the British education system. With sex education it can help lower the already falling pregnancy and abortion rates even further so they are closer to more advanced countries such as Sweden and the Netherlands. STI rates will decrease as people will understand the full impact of their effect and use a condom/femidom, as well as long term reversible contraception such as the implant or IUD. Respect for women should get better with less everyday sexism and sexual harassment cases as (predominantly) men do learn that this is truly unacceptable. Eating disorders could go down and body image issues could be tackled at the ages when they often start.
One such way is to make sex education compulsory for all schools. Sexpression:UK, a student organisation with branches at 30 UK Universities that deliver that all important sex education to local schools, are trying to change this with a petition to Scotland to implement sex education for all schools. Please support this, as any nationality can sign, and will make Scottish young people more prepared for the world.
Petition is here: http://www.scottish.parliament.uk/GettingInvolved/Petitions/MakeSREstatutoryInScotland
For English people, a collaboration of sexual health charities have a similar campaign under the Sex Education Forum. This has a petition for young people to sign but also a letter to send to your MP so please do have a look: http://www.sexeducationforum.org.uk/its-my-right