Or, it could be garden variety bullying.
Now, the school could be proactive since they have 'zero tolerance' on drugs, weapons, etc, but that can also sometimes end up meaning high threshold since they then just pretend not to see.
Or the school may just plain drop the ball like they often do.
On the college and student discussion, I recommend tight, defensive boxing to a draw, straight up (with the proviso of not taking a bunch of blows to the head during training, even with headgear). Here, it's a little trickier because you're his parent and he wants you to think well of him. And even with someone a person's same size, even with training and practice, going to win some fights and lose some fights. Actually prefer a draw if possible, because you are not trying to embarrassing someone you're likely to see again.
"Tight, defensive boxing to a draw. One week."
http://www.wrongplanet.net/postt134616.html
The point of boxing is that it gives a baseline of confidence and then you're hoping you don't have to use it. Hopefully, the zen of it all is if you're prepared, it's then less likely to happen.
Actually, it sounds like your son was pretty matter-of-fact about it. This clown tries to put something in the locker. Your son takes it out and puts it on the floor. Hopefully, this bully did not get the interaction he was expecting and will leave your son alone. Maybe, maybe not. Sometimes bullies get "stuck."
PS I AM NOT A PARENT. I am just someone who has lived life on the spectrum and I try and be a pretty good guy.
And I wish junior highs weren't such venues of bullying, but there are.