There are many sites out there that are not useful for your purposes, and class time is limited. Don't waste time having them evaluate resources in addition to having them perform a specific activity. If you need a place to research good student sites, go to the TeachWeb directory. Everything is organized by subject area. Do not let students visit sites that you have not approved for your specific activity.
Students waste a lot time searching for information.
You have much better time management and analytical abilities than your students. There is not enough classroom time generally to let them wander around the vast landscape of the Internet looking for tiny nuggets of information. Don't give them the chance to get off task. Even if there is filtering software installed, there are many ways students can waste time on the Internet, such as chatting and playing games. If you activity is to have them look for specific information using a search engine and web directory, make sure it is kid- friendly.