Anyone who has tried to show students how to use a GPS receiver before going into the field knows that there are certain things you must go over with them if they are to be successful. For example, if your receivers do not have a built-in electronic compass (which most affordable models do NOT), then you must tell them that they have to keep moving in order for the arrow pointer to continue pointing in the right direction. There are so many other things that can lead to student frustration quickly and, ultimately, lead away from the point of the lesson.
Take it from me, a teacher in the trenches who has seen Murphy's Law in effect on more than one occasion while teaching GPS to students: Try guiding them step-by-step, in a constructivist approach, and build upon basic skills. Do this while hunting for something very simple, like golf balls or tennis balls...easy to hide and not too difficult to find. Create an instruction sheet with an explanation of the receiver's main buttons, functions, and screens, as well as helpful tips. Below is a link to a document I made using the Garmin GPS60 receiver. Use it as a template for the receivers you use in order to make your GPS receivers more user-friendly to your students. Below that, is another great resource called Geocaching Cheat Sheets featured on http://issuu.com/lcisdtech.
Good luck and let the hunt for learning begin!