While I agree with fellow posters who bemoan the lack of leadership from parents, administrators, and coaches, I would like to think each individual player bears some responsibility as well.
Knowing the difference between right and wrong is something different than making the wrong choice. I think student athletes of that age and experience have experienced big games against fierce rivals where play gets chippy, and should know how to respond.
Adults who would condone this behavior should be identified and barred.
Right and wrong is pretty simple: if you have to stop and think about whether something is wrong or not, well, you have the answer right there.
That said, there seems to be an increase in reactionary responses, like another poster stated, because someone disrespected them. I guess letting your play do your talking is from a bygone era.
I just finished my 38th year of basketball and I think I've seen a shift toward less patience and more negative displays of emotion on the floor and in the gym.