In regards to a solution to keeping children safe online, and limiting web time. One quick solution at the network level may be to look at some newer routers. For less than about $80 there's a couple at Wal-Mart that have a lot of these features. Above is the price on Amazon you might compare if you take a look at the models they have on the shelf.
Here's is the overview/specifics sheet in PDF of the Norton Parental Control software that comes on the router;
http://www.belkin.com/support/dl/f5d7230_parentl_control.pdf
I would also certainly sign up for this service below from Norton and try it out first, it is free to use, while the router filtering service on the router above "protects" your kids from inappropriate sites, it didn't appear to have the time of day functions like this Norton Online Family does, which would allow more of the dedicated daily study time (no internet between 7PM an 9PM for example); both will give you reports for an additional fee, but this may be all you need right now;
https://onlinefamily.norton.com/familysafety/loginStart.fs
Be sure to watch the video on this page on the bottom right for an overview.
I believe it was on TV some evening this week and I only saw the end of it, all of it now, but it gives some insight into the trends we're seeing with students and technology lately, both positive and negative.
Watch Digital Nation on PBS. See more from FRONTLINE.