Unacceptable behavior is exactly what it is!
Cat, dog,whatever, the way in which the killing was done is unacceptable behavior. One does not take an animal and cut it's head off for fun! This is sick behavior, those boys should be examined by a doctor . They need pychological help! These are grown men!
I have known guys at my husband's work that use to use stray cats as target practice. I threatened to call the police and they quit.
This has (almost) nothing to do with animal rights, these types of people are sick. It is not normal to want to kill or hurt living creatures other than for the sole aspect of hunting for food ( in which true hunters use weapons that inflict very little pain and kill instantly).

As for the school, they should have enough sense and dignity to identify that these students are mentally sick and kick their ugly *&$#* out!!!!