How many people believe this makes the world a better place? A company called TenNine has hung hoardings in the corridors and common rooms of 750 British schools. Among its clients are Nike, Adidas, Orange, Tesco and Unilever. It boasts that its "high impact platform delivers right to the heart of the 11-18 year old market".

Other firms are closing in. Boomerang Media, which represents Sega, Atari, Virgin, Umbro and others, has persuaded schools to distribute Revlon perfume samples to their pupils. This campaign, it says, "was effectively linked into their PSHE and PE classes". PSHE means personal, social, health and economic education, or "learning to live life well". How the disbursement of perfume by teachers helps children to keep fit and live well is a mystery I will leave you to ponder.

Advertising in schools offers corporations a genuine captive market. Trade associations which defend the dark arts of persuasion argue that if you don't like advertisements, then you don't have to look at them. But in this case you do. While surveys suggest that roadside hoardings raise awareness of a company's products among 28% of the people who pass them, posters in schools, according to TenNine, reach over 80%.

Every year, advertisers press a little further into our lives, shrinking the uncontaminated space in which we may live. In ways of which we are often scarcely aware, they change our perceptions of the world, alter our values, infiltrate the language.

But at least adults have some defences. As the advertising executive Alex Bogusky points out, "children are not small grownups. Their brains are fundamentally different, the big difference being that right-hemisphere brain development doesn't really kick in until the age of 12. This is important because without the right hemisphere involved, all decisions and concepts are very black and white. If you sit with a child and watch TV commercials, you will notice how vigorously effective the messages are. 'I want that.' 'Can I have that?' 'I need that.' These words come out of their mouths with seemingly every message, and they mean it and they believe it and they are defenceless against it."

These defenceless people are being pursued with precision and ruthlessness, and governments fail to protect them. In the United Kingdom advertisements for food high in fat, sugar or salt cannot be broadcast during children's television programmes. But they can be fired at children from UK websites. Nestlé, for example, has a "family" site in which children can "explore the fun" with Quicky the Nesquik bunny. There are prizes to win, jokes to share, games to play and TV adverts to watch. Cheestrings, making no pretence of reaching anyone other than young children, has a UK site called "101 fun things to do before you are 11½". Among them is "get fit fast", illustrated with the fatty, salty Cheestrings mascot.

The Sugar Puffs site, plainly targeted at children, invites them to "join in the fun", with jokes, computer games, electronic postcards to send to their friends and television ads promoting the junk the company sells. Disgracefully for the Variety Club charity, there's a cross-promotional page which uses the charity to push the Sugar Puffs brand. Surely the Variety Club is supposed to help sick children.

In his book Childhood Under Siege, Joel Bakan shows how computer games and social networking are being merged to create new advertising platforms. The aim, according to an executive he quotes, is to "get users in the door to play for free and then monetise the hell out of them once they're hooked". One way is to issue points or virtual coinage to kids who click on advertisements.

All this is promoted as fun and freedom. Parents who try to restrict children's access look like prudes and killjoys. "As our kids become immersed," Bakan notes, "in a [corporate] culture that works to pry them loose from us, we become less able to find the connection, respect, authority and credibility we need to keep them safe, healthy and in the long term happy."

In its coalition agreement, the government promised to "crack down on irresponsible advertising and marketing, especially to children". It commissioned a review that concluded that if advertisers failed to regulate themselves, the government should take action. The government says it agrees with "the thrust of all [the] recommendations".

So where's the action? There's a website making it easier for parents to complain, and a promise by advertisers, already broken, not to use children as brand ambassadors or for peer-to-peer marketing. But on issues such as advertising in schools and the online promotion of junk food, not a word.

So it didn't take me long to decide to sign the open letter by a new campaign called Leave Our Kids Alone, asking for a ban on all advertising aimed at children under 11. It is long overdue: it's a marvel that we have for so long tolerated this capture of children's minds by companies exploiting their innocence and wonder. This is a campaign about more than advertising. It's about who we are: free-thinking citizens, raised on the best information and judgment that parents and teachers can provide; or captive consumers, suckled at home and at school on subtle corporate lies. I urge you to join it.

Twitter: @georgemonbiot

A fully referenced version of this article can be found at Monbiot.com

• A letter from TenNine can be read here