In the process of looking for a tool to check the accessibility of a form the other day, I came across this list of online tools and browser plugins.
In addition to web page validators, there are a number of other useful tools on the list that give you an idea of how you pages will work for someone using a screenreader or someone with colour blindness.
Top 10 accessibility tools - article by abilitynet.org
The Abilitynet site contains a wealth of articles, advice and resources for anyone looking to make their web content as accessible as possible.
Abilitynet website - online accessibility specialists
The section on legal compliance in particular caught my eye.
What the law says about accessibility in education - an overview by abilitynet.org