Before I toss in my 2 cents, a couple of questions. Is this for high school or college? Will it be used in the classroom, or merely something for them to look into? Are these students are are looking at theatre for a career, or hobbyists trying to learn as much as they can? Thanks.

DoubleCad by IMSI/Design is free and is a lot like AutoCad LT. It is a bit more time consuming to learn, but once you get used to the interface it is quite nice. It is basically just a drafting program, but one can create templates and once you get comfortable using the program you can create blocks for just about anything you use frequently; platforms, walls, lighting fixtures, etc.

I never met a CAD program I liked, though Im devoted to doing 3-D with Blender someday.

I think maybe you can find something usable at:

http://www.stagelink.com/resources/Software/Lighting/index.html

Hope this helps,

Scott - http://www.stagehand.us

Let me know what workz good for you and your students.