maxwell morton , 16 , from pennsylvania , will be tried as an adult for shooting ryan mangan .
mother of morton 's friend told police her son received snapchat image of him with victim - message had maxwell written across it .
he admitted to shooting mangan after police found 9 mm handgun hidden in his house .
both teens were juniors at jeannette high school .
morton was also charged with first-degree murder and one count of possession of a firearm by a minor .
morton 's lawyer and family said the two boys were friends and there was no bad blood between them .