There is more to Google than I know what to do with but one way to help refine your student's searches when they are given an assignment is to create your own mini search engine with Google Custom Search. This allows you to select the sites that you want students (or anyone else that looks at it) to use when looking for information on a particular topic. You can set up the search engine so that it only uses the sites you choose, or gives preference to those sites.
By clicking the "more" link on the Google homepage (or your iGoogle page) and then clicking "even more" you will find yourself with quite a spread of Google products and innovations. It is here that you will find the Custom Search option under the banner of "Explore and innovate". Setting up your own search engine is then quite simple. Fill in the name, description and keywords for your search engine and then select the sites you want that engine to search. The Standard edition also has the option to not show ads on the results pages if you are a non-profit agency, University or government agency. My first offering is here or use the search box below: