This is a good overview at the W3C: Improving the Accessibility of Your Web Site. It’s easy reading and clearly written. It would make a good starting point for a classroom discussion on retrofitting for accessibility.
Auditing your website for accessibility
A couple of articles by my friend David Moore at iCubed provide a plan for evaluating a site for accessibility. David’s done this professionally in the the U.S. and in Ireland, and the article gives excellent advice about what to look for and check. David’s tips go hand in hand with the W3C overview I mentioned above.