Some good responses and have to agree, IMO that the bat TYPE(characteristics) should be more emphasized than the bat Brand(popular)...ie; balanced, end loaded, stiff handle, flexible handle, composite, alloy etc... Then focus on the weight/length issue. My middle son is 13, its his third season on the big field, second season he will swing a -3, occasionally using a -5 if we come across a real heater, but haven't seen one yet for this age group in our area. He actually decided to swing primarily with wood this season, I told him I have no problem with that and was happy he made that choice.
Someone mentioned the problem they see is kids swinging bats that are to heavy, the biggest problem I see is that they all want to swing bats that are to long.
I also agree that unless they are really small, or maybe even first year on the big field, they should be swinging at least a -5, but really should be swinging a -3 if they are in 8th grade.