I was in the Keys two weeks ago and did penetration dives on both these wrecks. It was my first time diving both of them so I didn't do anything extensive but I'm guessing if you're teaching students you won't want to go too far in.
Spiegel Grove: Penetrating the 2nd deck of the main superstructure (at about 80ft), from the entrances towards the aft side (by the flag) worked well. The corridors here go in a 'square' around the entire deck with lots of little rooms off both sides. There is also a large rope / guideline down one section. Natural light is visible at all times and there are access holes cut into most of the rooms. There is a lot to see including tables, cookers, cabinets, bathrooms and even toilet cubicles! There was very little silt on this deck and there was ample space to access each room safely.
Duane: I did not do much penetration on this wreck as the current was ripping and I used up more air than I planned getting down. There are a lot of good swim throughs on the upper decks allowing you to look down into the lower decks. There is also a long corridor that can be accessed from a door on the starboard aft side of the wreck (at about 100ft), it runs all the way to a hatch closest to the bow on the starboard side. This wreck was a lot darker inside than the Spiegel Grove but prettier in my opinion (probably because it has been down a lot longer).
Please know I am by no means an expert on these wrecks having only dived them the once. I'm sure you can get much more detailed info from others on the forum but I hope this helps give you some ideas.
Have a great dive!