If one reads the updated article, it reads one of the boys with the last name "Anger" (yes, that is his last name) was the one who dropped the 20 lb. stone that killed.
We don't know if that is what happened definitely until a trial but I have a feeling, one of the lads probably felt guilty or something and turned himself in. Thus, I think there's a chance, some of these boys who did not do the actual killing "could" get some leniency for testifying.
Obviously, a trial is needed to find out what happened.