The use of a Variac is strongly recommended. Set it to about 50 volts initially, and show the class that the ring climbs up the core a short distance. This is the principle used in "MAGLEV" trains. Then shut the switch off, turn the Variac to 110 volts or higher, and flip the switch on. The surprise factor is always a key factor here.
You may want to have a ring with a break in it available. This will further prove Lenz's Law by showing that the conducting loop must in fact be closed for the current to appear in opposition to the change in flux. The voltage across the break can be read to check Faraday's Law. You can also put a multi-turn coil around the primary coil with a bulb attached and see it light up. There is one ring which is nearly completely slitted -- at the remaining small "bridge" of conductor, very large currents flow, and it gets hot very quickly.