Ask a student what their most difficult day was on skis is and most will tell you their first. In addition to dealing with gravity, there are these four-foot long planks on their feet, skewers attached to their wrists, and boots only Frankenstein could love. Fortunately, the first day goes fast.
After explaining some of the basics about the skis (edges, tips, tails, bindings, and ski brakes) it's important to simplify skiing down to its basic concepts: tipping, turning, steering and weighting. Have all of the students put on their right ski first and explain to them that it's like riding a skateboard. However, on skis, you can tip or lean forward or backward, turn by tipping onto the ski edges, and steer as if their skis were on the end of an axle (their leg) and weighting and un-weighting - lifting one foot up at a time.
After walking around in a figure eight to learn basic control of the equipment, have the students put on the second ski and see if they can accomplish the same maneuvers without falling. If they do fall, it's a good time to demonstrate to them how to get up and put their skis back on.
Finally, find a very short, very gentle bump and ask your students to learn to glide and stop using the "pizza" and "french fries" techniques. French fries means skis parallel to allow them to track straight and build speed, while a pizza (pointy end to the front) will slow them down and bring them to a stop.
Then it's like the best way to Carnegie Hall: practice, practice, practice.