I teach Design in grades 8 to 12. I currently run 3 Linux labs and servers and will be doing a lab up in Win XP in a month or so. I use an older app called CadKey 19 for Windows and run it in Linux perfectly. The program is rock solid and very powerful for such a small install. It is parametrics based and 2D/3D. Student purchases are often very inexpensive. It has recently been purchased by a Japanese company called KeyCad. I used CadKey for my Garage Build Drawing: http://www.jeepforum.com/forum/f37/n...94/index2.html
Low down on this page is my CadKey design.
Free is difficult... QCAD is 2D and used to be a free package if I remember but now seems to have a fee. A purchase price of 44 Euros doesn't seem too much and you can load a demo to try out.
Google Sketchup is free for windows and may do what you want.