Tadpole, good to see you.
I'm in no way defending the students who participated in the stoning of this girl, but it seems to be a case of "a little knowledge is a dangerous thing."
If the students had known all the facts regarding the participation of fasting during Ramadan, then this probably wouldn't have happened as there are exclusions to fasting, which can be of a personal or medical nature.
As for the cruelty of children however, I give you the following example. In the schoolyard whenever there was a fight, the kids would circle around and egg on the participants, or taunt one and encourage the other, here in the USA. The spectators seemed to feed off the energy of each other, becoming frenzied like piranhas in bloody water. Eventually a teacher or aide would come break it up and things would abruptly go back to normal.
I'm pretty sure that it was kind of the same thing, with an underlearned religious conviction thrown into the mix.
But, yes, parents and religious leaders do need to start teaching peaceful alternatives in conflict resolution. Stoning someone because they piss you off isn't the answer.