Make sure that the latest Firmware is installed on the GPS.
Make sure that the GPS Datum and paper map Datum are in agreement.
Make sure the batteries are fresh or are fully recharged before going on a wilderness expedition and always carry some spare batteries.
Make sure that the GPS Magnetic and Grid deviation is correctly set within the GPS to the actual Grid Magnetic variation for your geographical position i.e. such as locations near to the magnetic poles can vary over very short differences in geographical location.
Never trust your life to a GPS to get you out of a fix. GPS is very accurate 99.9% of the time but the 0.01% of the time the GPS may be some furlongs or even hundreds of miles out. Use a map and compass so that you never end up getting into a fix in the first place.
Never trust a GPS to point you in the right direction - they aren't very good at this.
Learn how to use a map and compass properly, how visualize the terrain from the map contours, know how far the distance is between each of your own walking paces, how to count etc - its the only way you can appreciate how to use a GPS properly and safeguard yourself against all the things that can go wrong with the GPS reciever and the NAVSTAR satellite constellation.
If you become really proficient at map and compass work you can then just use the GPS for fun and trivial activities such as marking out way points then uploading them into Google earth.
You can even then decide to leave the GPS at home if you wanted to.
You can even have some fun with some of those cheap Chinese GPS jammers when teaching your class.
And lastly; Using a GPS is navigational cheating, if you have to constantly use one in the wilderness then you should think again whether you should be there in the first place without the appropriate skills i.e. not knowing how to use a map and compass.