One thing that you might try to do in an essay like this is to argue that the time that teens spend in these ways is actually beneficial to them. This argument would be more effective with reference to the computer screen than the TV. With computers, you could argue that teens are developing the sort of easy familiarity with technology that they are going to need if they are to succeed in a time when technology grows in leaps and bounds and so many of the best jobs are related to technology.
As for TV, you might argue that teens today have stressful lives and they need some time to themselves to unwind. TV can provide that just as it provides adults with a way of relaxing. You might also argue, as the link below does, that watching TV helps teens cope with issues that they are going through and that they see mirrored on the screen.