Truth be told mate you have possibly the worst age group of any sport. Too many distractions, drink, girls and girls and drink and girls. I coach U12 GAA but have involvement at u16 too.
The advice given so far is sound - never ball out an individual, always encourage. Identify short comings and deal with them, don't point them out.
Stop the snipers, you know, the ones that think they know it all and have no problem shooting down players of lesser ability
Training can be fun and if they are having fun they will train harder. Always take an opportunity to steal, I mean observe, drills from other coaches and age groups. The coach that knows it all hasn't been born yet.
Be aware that you are dealing with minors and its a sad indictment of todays society, but never get isolated with a minor. Its not child protection its adult protection. Commonsense at all times