Dear all,
Sorry about joining this thread so late!
I think that DragMath is only half the story really. Once you have your student give you a mathematical answer you will want to do things with it! This is what I also designed STACK for. Indeed, STACK came first, and then Alex and I worked on DragMath together to create a formula editor.
http://www.stack.bham.ac.uk/wiki contains much more information.
Chris