So many teens (unfairly) get a bad reputation. Most are great. But even among good kids, from early adolescejce, there is immense social pressure to "not be a snitch" and turn a blind eye to criminal behavior. Snitches get stitches, after all. Some even celebrate and glamorize the thugs. Sometimes out of fear or coercion, but many think it's "cool" to be a hoodlum and tough. 

But yes, it's baffling as to why all accomplices (that can be proven) aren't being charged. Perhaps there is debate about whether they should be charged as juveniles or adults, to what degree of charge should be filed, or whether others involved were actually willing participants or threatened into aiding. Just some possible explanations.