This is an easy quick activity to brush up on finding points someone else has already saved or points you had saved before. A deck of cards is a nice tool but any numbered sheets of paper, or other unique trinkets would work as well.
Many easy variations could include the teacher going through an interpretive walk around your school grounds and uploading saved points to a computer using the DNRGarmin free tool. (You will need the download cable to do this.) Then you can upload the points back to the rest of the GPS units before sending your class out with a worksheet of activity descriptions of what to do at each of the sites you have chosen.
There are many other variations that would be good to try after this. Geocaching is really just a form of hide and seek. A favorite alternate is to pick out any number of locations and write a riddle or poem or some clue about what it is in each spot that you want students to look for or describe or take its picture. On a sheet with the clues each student must write what they think the clue described on a worksheet you prepare for them.