I have also written a blogpost for the IEA on the subject here.
The ban itself is uncontroversial and inconsequential—the headteacher seemed perplexed by the media attention and conceded that he knew of very few pupils who use e-cigarettes—but the reasons given for it are more concerning. The letter sent to parents claimed that e-cigarettes "may be acting as a gateway into smoking, rather than a way of stopping". In other words, non-smokers start using e-cigarettes and then progress to smoking. E-cigarettes are therefore part of the problem rather than part of the solution.
Do go read the whole thing.