A zero tolerance attitude is (in my opinion) wrong - every case is different and judgement needs to be exercised.
Personally, I can't see many kids wishing to emulate a Sikh (wearing turbans etc) so that they can carry a knife underneath their clothing (and the minute it is exposed is the minute you realise that they are not a Sikh).
If this does become an issue, then fair enough, and in some inner city schools, I concede that it may be an issue - but in the majority of cases, I can't see it as a problem.
Do you get worried when you see a turban that the man wearing it has a knife on him?