I was also a US Army instructor - for 4 years on active duty, plus 8 years in the Army Reserves.
As LT Dan stated, the "Map Reading & Land Navigation" courses (usually 8 hours classroom and a 4 hour practical exercise) was most challenging to teach, and most difficult for students to learn.
Other instructors frequently asked me to take over "Senior Instructor" duties for them, while they served as assistant instructor. Since my branch was Field Artillery, I was accustomed to being sure I knew where the rounds were going to land before initiating firing commands.
That said, I always used 2 compasses, AND a topo map, in conjunction with each other. In teaching students to use a compass, I included the precaution to be aware of magnetic influences, such as the presence of iron objects (rifle, pistol, trucks, railroad tracks, overhead power lines, hand grenades hanging off your web gear, etc.) The army even specified exact "standoff distances" for each object, and included several questions in the end of course test to be sure the students understood/learned that a compass needle is sensitive to magnetic distractions, some of which are not obvious.
As far as iron ore soil/rocks, underfoot, you can't standoff from such being underfoot. That's when your topo map is your best hope. "Don't leave home without it" is a good rule of thumb.
Now I must humble myself a bit.
I was hunting in thick cover in the East Texas Piney Woods, a mile or so from the nearest road, following a creek bed, and decided to walk back to the road on a straight line, using my compass. It was cold enough that I was wearing a pair of mittens over a pair of gloves. The mittens had a feature that allowed you to fold back the part that covered the fingers, thus allowing greater dexterity with the fingers. When folding back the finger covering, a magnet in the finger covering clung to another magnet on the back of the hand.
So, I uncovered my fingers to more easily manipulate the compass, and proceeded to march straight toward the road, stopping every 50 paces to hang a blaze orange piece of surveyors tape on a tree, so I could return later and set up a tree stand. After marking off about 4 such waypoints, I looked at my back trail and noted that I had formed an erratic zigzag with my markers, rather than the straight line I had expected.
First, I checked my 2 compasses against each other, and they were erratic. Then I shot a back azimuth from my location to the next marker.It was wrong, with both compasses. Finally, I carried my rifle (a possible magnetic distraction)10 yards off the trail, then went back to my last marker and measured the back azimuth with both compasses. Every azimuth I measured, both compasses, was erratic.
Searching for other magnetic distractions, I discovered the magnets in my mittens. I took the mittens off, put them with my rifle, 10 yards away, then took azimuth readings. Problem solved!
I then went back to my first waypoint, collecting all of my markers, and proceeded to lay out a straight line to the road.
Lesson learned.
The magnetic mittens stayed in the woods!
Ten yards off the trail!