In general there is nothing wrong with this, I think it's another sign that the school system is out of control, and has been getting increasingly worse since columbine. The policies that lead to this kind of thing are basically a way of trying to sidestep the actual issues at the heart of the matter.
Let's be honest, there have been things like music, movies, and other things all about kids rebelling against school authority, and even doing things like killing their teachers, blowing up schools, and all kinds of things. It's all in good fun, and nobody takes it especially seriously.
I mean crap, the old
"Glory, Glory Hallujah, my teacher hit me with a ruler"
"I hid behind the door with a loaded .44 only saw my teacher once more"
"went to school the next morning, my teacher wasn't there"
"so I went to the hospital, and shot her in the underwear"
is an institution.
Then of course we have "On top of old smokey, all covered with blood" which ends with a kid throwing a grenade and blowing up the teacher AND her grave.
The point here is that these songs are intended to be kind of F@cked up, and darkly amusing to students, but nobody takes them seriously. Heck the first one was taught to a class I was in by a teacher who was making a sort of joke about the whole thing and showing how long it had ben around (going back to like when corperal punishment was allowed in schools).
Now, I can understand how things like this are disturbing in light of things like "Columbine" but they aren't what's at fault. It's also no worse than a lot of other messed up children's songs which are about things like the bloody plague ("Ring Around The Rosie").
The fact that the kid was in therapy is also an issue, since I can't help but wonder what kind of disclosure/confidentiality agreements this violated with someone even finding out about it. It also occurs to me that if the kid has issues to be in therepy to begin with, they should have been asking why he hated his teacher that much, and truthfully this kind of reaction almost guarantees your going to have issues with ANY kid in school wanting to open up to a therapist about anything important. I mean there is a bond of trust there which is why there are supposed to be rules about disclosure, you start arresting patient/clients for stupid things like this and good luck with the other students when it matters. Even if this kid is unusually messed up, bring another one is for something else (sexual abuse, drug problems, gang concerns, whatever) and if they refuse to say anything or work through their problems, the only ones the system is going to have to blame is itself. Makes me wonder if people in the school will die now from issues that could have been prevented or worked through if people trusted the therapist... it would be ironic.