Given that you're young and you want to expand your repertoire of training outside of Boxing, watch these Russian kids do their thing:
I wont suggest you try to backflip off heights, but the hopping and quick climbing up walls, spring up and bar swings will definitely quicken you. Much of it is plyometric, and dont think that the moves require much training either, it is much easier than it looks.
Agility is not only about size and speed, it is about building mental coordination and flexibility. In gymnasts, the rear lobe of the brain which is responsible for balance coordination is much more developed than the average person. A lot of that contributes to your ability to change direction quickly.
'Urban Ninja' is the secret to developing exceptional agility -in my case anyway.