The problem in the past by having students search on Google was that they would use general terms and end up with many sites to sort through. Now, Google has solved that problem by creating Google Co-op.
Google Co-op allows the teacher to choose which sites Google searches from. It even allows you to post the code to the search engine to your web site or blog. There is also an option to invite others to contribute to your search engine, and you can use the Google Marker to bookmark sites directly to your customized search too.