They are currently legally sold to under 18s, and are often cheaper than traditional cigarettes.
Although they do not contain the harmful tar found in normal cigarettes, experts fear they could encourage young people to swap to tobacco and many schools have already banned them.
One of them, Highfield Humanities College in Blackpool, does not allow them on the premises.
Deputy head Sara Dewhurst explained: "The reason for the ban is over concerns that pupils could be encouraged to start smoking."
Andy Sievewright, from Acton High School in West London last year wrote to parents: "Young people who use e-cigarettes could easily become addicted to nicotine and progress to smoking."
E-cigarettes have been widely used by celebrities, with fans apparently including Leonardo DiCaprio, Kate Moss, Lily Allen and Cheryl Cole.
And the sweet flavours are said to add to the appeal.
Ella Williams, 18, from Dorchester, Dorset, told the Sun: "It's a social thing.
"Everyone was talking about e-cigarettes so I tried one of my brother's.
"It was strawberry flavour which I liked. I've also tried the menthol one.
"E-cigs are cool because you can smoke them indoors and there is no smell to them, so it's good to do with your friends on a night out."
An American study of nearly 40,000 youths found that teenagers who had tried an e-cigarette at least once were six times more likely to try tobacco.
Dr Ram Moorthy, a spokesman for the British Medical Association, said yesterday: "Ultimately these devices do contain nicotine and young people smoking them are developing a habit which they could potentially be stuck with for life.
"It is also concerning that these devices so closely mimic cigarettes, which is normalising smoking just when the smoking ban has done so much to make it socially unacceptable.
"There have been very worrying reports that teenagers have been asking for e-cigarettes for Christmas presents and that they have become commonplace in the playground."
He added: "We want to see legislation banning e-cigarette sales to under-18s brought in as soon as possible, and see it strongly enforced."
- Daily Mail