I'd definitely ixnay the duct tape. Aside from not being designed for human skin, some of the chemicals in the adhesives can interact badly with human skin...as I found out on one of my own misadventures in moving...LOL!
Something I'd personally recommend is the rlp's suggestion of athletic tape. This stuff is useful for a lot of things...so much so that I keep it in my backpack when I hike, and I'm loathe to carry anything with me on a hike that I don't absolutely need.
Aside from coming on and off the skin with relative ease, it can tape up training ouchies easily. It also does a great job of holding a hot pack/cold pack/bandage in place, or tape down piercings, wedding rings, and other forms of jewelry that might interfere with training but the student may be reluctant to take off.
It can also be used on mats, the floor, or gear, without leaving a lot of gunk behind. I remember working a stick drill with a partner (very senior guy) and my final strike (an overhead) seemed to be all over the place. My partner put a piece of athletic tape on the center of his stick. He pointed to it and instrcuted me to "Hit here," which I did. Drill much improved. You could do similar things with focus pads, lines on the floor, or other forms of training. Masking tape may be cheaper for stuff like this...but when it comes down to using stuff you have around, I found that I had athletic tape around when I needed it more often than the other stuff.
