Having two kids myself, both using their computers a lot, my main concern is not academic performance (in fact they are both top of their class despite playing counter-strike and other games for hours each day). I am a lot more worried about them not getting enough exercise, which in the long term will have fatal consequences.
The discussion on parental control or kids self-control is uninteresting. What is interesting is making them interested in and getting an understanding of what is good for them. That takes a lot more than just telling them "You cant play any more today".