Is that more important than the safety of students? Clearly it's not on aeroplanes, where Sikhs comply with safety standards and remove their kirpans. Other sikh kids at this school do not wear kirpans. It's called compromise.
Where would you draw the line with this kind of thing? Would you, as someone else said, allow kids to bring loaded guns to school, if they were "symbols" against oppression? What if I have an axe that symbolises the hard work and perseverance of my lumberjack ancestors, can I not carry that around with me as a 'symbol'.
Of course he can. His religion may forbid it, but that doesn't physically stop him from using it as a weapon. Not to mention the fact that someone else could take it from him and use it as a weapon.