It seems to me like you're talking about a good rule, not a good law. An individual teacher, or even an individual school district, absolutely should be able to say students can't use the Internet for personal purposes during class time. But I don't see the need for federal legislation mandating that be the case everywhere.

And I can even think of legitimate academic uses for social networking sites -- looking at TWoP to understand criticism, for example, or viewing political candidates' MySpace pages in a civics class. There's no reason to make the entire site off-limits!

[Edited to fix HTML.]