You can’t carry a knife for self defence. You can carry one if you have another reason to carry it.
As very few teenage boys are fishermen, leather workers, or similar, police will tend to assume they don’t have a good reason to be carrying a knife unless they can explain it.
I carry a pocket knife everywhere, and if I am asked why - as I have been once when he pinged a metal detector, I simply say it’s a useful tool that I got used to carrying in the navy and there’s no problem. A teenage boy might not get as easy treatment as I do.