You know I couldn't let this go without suggesting Tk, as a way to introduce them to graphics. This node, Tk Patio/Office layout designer may be low overhead enough for them to understand, yet have enough fun and wow! factor to interest young students.

Another great one to play with, is SuperFormula, see SuperFormula with gnuplot and Tk.

Finally, if you want a nicer higher overhead canvas, with more features like semi-transparency, zoom, and rotations, see Tk::Zinc. I have a little ready made tutorial for beginners at illustrated Zinc tutorial . There is even a Zinc version of Pong there. :-)