On the one hand, I think that young people today carry kanives for self-protection. According to a recent survey, the most part of Spanish population feel that they aren't protected enough and don't trust justice. Anna, a lawyer, declared: "People are actually disappointed with Spanish justice, since we have a system that condemns shoplifters to several years of prison while a lot of murderers are in the street, living together with our children".
On the other hand, I believe that having seen that justice isn't very fair, teenagers have decided to protect their own lives by themselves, and that's why they usually have guns. However, there is another kind of population that carry knives not only to self-defense, but also to riot.
Taking all this into account, in my opinion, police should check people's clothes and bags in public places, such as concerts or big parties, above all, in places where alcohol is consumed. But what's more important is that they should try to put an end to this problem by controlling the shops where guns are sold.