Problem with Prison sentences for carrying knives is two fold as I see it
a) They're likely to learn even more bad things to get involved in and how not to get caught doing it..
b) The kind of people that (predominantly) carry knives will just see it as a badge of honor anyway so it's no deterrent.
I have to admit my desparation and frustration at the whole issue leads me to thoughts along the lines of....
a) Replace prison terms with a tour of duty in somewhere like Iraq or Afganistan for anyone caught carrying an knife. The threat of real death, (in circumstances where they feel disempowered - knife carrying is significantly about being "the man" on the streets), might just create enough fear to discourage it.
b) Punish those caught with the person who actually has the knife (say 3-6 months inside), might just start a wave of peer pressure as a significant number of people will not want to "go down" for something their "mate" has done.
c) Stop wasting public money on putting people who stab/shoot/kill commit any kind of violent act against another person in prison. The French had the right idea. Find a deserted island surrounded by shark infested waters and dump them there and let them get on with it. If they want to live by the rules of the jungle let them, but deny them the right to do it here.
Saying those things doesn't make me feel comfortable but I'm getting older and more reactionary each year.