You could use javascript and html. There's a port of processing to javascript, so you know that it is powerful enough for the things that your class will cover. You can author html without knowing a single thing about what it looks like. In fact that is the preferred way to author html.
The main downside of javascript is not javascript itself, but the browser dom. That is the interface into controlling the html elements. However, a library like jquery, or mootools, or dojo can take care of most of those problems.
As for accessiblity, have a look at WAI ARIA also opera's intro to WAI ARIA.a
WAI ARIA is a way to build rich javascript applications while playing nice with screen readers. It's very cool. I've not seen more work and passion put into making the web stack accessible in any other programming stack.
