VEX CNC

My highschool science club is the proud owner of a couple VEX kits -
erector sets with brains. We are thinking of making a CNC project
with this VEX - more as a learning exercise than functional. We would
probably just use a Sharpie pen on paper, or something like that. The
motors are controlled by a servo PWM signal as we understand it, and
we can create autonomous functions. However, that programming is done
in C, and I can see where we could make a square, or even a slanted
line, maybe, but I'm puzzled as to how to make, say a circle or
something more complex than a straight line. Are we going to have
more problems than it is worth, and learn mostly that we need to
finish our stepper controller, and use a CAD/CAM program to do this,
or mgiht this be an interesting project, if the mentor just knew what
he was doing?? Thanks, Chuck