The robot uses a pen to draw as it moves. The wheels control the position and the servo can lift the pen up/down when no drawing should occur.

This is my fist take on this robot. I'll let the students have a stab at the code and put that on GitHub once we have something sensible to move it around with. The hardware was dictated by what the students already have in their kits, but this is probably one of the cheapest robot platforms out there.

Regarding precision - it's not crazy good, but more than good enough for my purposes. See this video for an idea of what to expect  better the pen fits, the more repeatable the drawings will be.