No it didn't. Ya better read more carefully. From the article below;
The four teens, all aged 13 or 14, pleaded guilty to either murder or manslaughter. The boy who dropped the sandbag pleaded guilty to murder while the other three pleaded to involuntary manslaughter
Why did the 3 get accused of Manslaughter ...for what watching the one drop the bag?
Read the article. Clearly one suspect admitted dropping the sandbag and was charged with murder. The others were in on the 'caper' and were likely also throwing objects, but only ONE hit the mark.
This is not real complex.
Read the first sentence.