Knife crime won't be stopped by token gestures, but through more police on the streets.
A lot of kids carry knives simply because they feel vulnerable. I was in primary school and one day a bully threatened to beat me up. The next day I took a pocket knife to school to make sure of he tried anything, i'd be able to defend myself. It was incredibly foolish, but at the time I felt vulnerable, didn't think asking for support from teachers or my parents was an acceptable thing to do - so I thought i'll deal with this older kid myself. I was 9 or 10 at the time, but in our ends, that's how it was. Nothing happened, I put the knife back home and have only told this story to a handful of people. Even my family don't know to this day. If kids feel that they are under threat from people from knives, they will foolishly carry knives themselves to defend themselves.
Another way an increased police presence will help is as a preventative measure. Through the extensive use of stop and search, people carrying knives will be caught and sentenced to prison. Once the frequency of this increases, people will stop carrying them casually, but rather only when there is an intention to use one - as the risk of carrying one will increase. I live in a pretty rough part of my city, sometimes i can go days or a weeks without seeing a police officer walking the streets. You'll see patrol cars - but officers on the street are a rarity.
The next thing the police can do with increased numbers is target the gangs. These kids aren't stabbing for fun - they're doing it because they're in gangs that demand it from them. The gangs groom poor kids, or coerce them though threats of violence to do things for them. Hide drugs, deliver packages, look out for xyz. Once the kid starts being cooperative they treat them nicer and reward them for these small favours. Then the small favours become bigger and at this point the kid is in the gang, backing out of it puts them in a "with us or against us" situation and they fear for their lives and the lives of their loved ones. If the gangs are actively targeted they can't operate as as freely.
For me though policing is 50% of the solution - the other 50% is youth facilities. When I was growing up there were youth centres everywhere, they'd organise sports, or games or just be an area where kids could go for free, be amongst friends and chill in a supervised environment outside the home. These places usually were free of drugs and gangs, those sorts were kept away by the staff and police if needs be. Today these facilities are gone. When the youth are outside of their homes, they're might go to the park or other public spaces, the gangs also freely operate in these spaces. If there were youth centres and a greater police presence in parks (as well as park rangers, remember them?) then these areas would be off limits to gangs and young people could be outdoors without being easily accessible to gang members.
Finally a left field suggestion i read somewhere - metal detectors outside shopping centres, food courts, bars, public transport. Anyone who is aged between 12-25 and is not accompanied by an adult should be required to go through these scanners. Don't place these everywhere, but in areas where knife crime has occurred or in localities where it is likely to occur.
