I teach beginners and do a four block sampler with rail fence, pinwheel, ninepatch, and log cabin. If only doing one block , imo, it would definitely be the rail fence. I personally would do a beginners class on "How Well Do You Know Your Machine?"
Most machines are simple enough to figure out, and you would be doing yourself a favor in the long run. You would also do a little rotary cutting along with it, to get them started. Then send then home to practice their 1/4" seam before offering the quilt class. Has worked for me for 15 years now.
Good luck. Teaching beginners is wonderful!