Most people get easily distracted because they fail to control their own mind.
Meditation is a great way to learn how to focus, as well as put yourself into a zen-like state. Start out in a quiet place and then work your way up to being able to stand on a street corner downtown and block out everything except one sound which you're focused on. Along with that, training yourself to quiet the mind and maintain a meditative state naturally is another goal to accomplish not only for focus, but also as part of gaining and maintaining inner peace. I've found that most people have very busy minds. They have "random thoughts" (often repetitive) running through their mind all of the time; yet the person doesn't realize they are the thinker of those thoughts and that they can control them; but in order for them to control their own mind and thoughts, they must first become aware that they can control their mind, if they choose to, which most people do not.
So it doesn't surprise me that people get easily distracted because they don't control their own mind, their own thoughts; so any stimulus (such as a "gun shirt") is going to distract them. It also doesn't help matters that technology has contributed to this lack of focus with 140 characters on Twitter, Instant-Access-To-Everything, facebook and other social networking websites, data aggregation and self-delivery services such as RSS, etc.
Perhaps if the school wants total control over how people dress, they should enforce a school uniform policy and make the parents pay for them; however, that still won't do anything for their lack of focus and control over their thoughts and mind. A school uniform policy will only serve to remove one possible stimulus, not solve any problems. Other areas are like that too.... people thinking Gun Control will solve people problems, when in fact that won't do anything towards that goal.