are these values in pixels? might be worth mentioning... and I know you plan to revisit this, but maybe just a sentence in your example intro about the coordinates and radius again mean, like:
For example, you want your students to drag the Iceland label to an area near the upper-left corner of your image and drag a Sweden label near the upper-right corner of a 400 pixel wide x 600 pixel high image. You determine where each label belongs on the image, and the size of the area around each point that you will accept as correct. The code might resemble the following (where 2 is the ID for the Sweden label):