I agree with the comments regarding the 172 being reluctant to spin, but remember this is 2 up and in the utility category. The CofG is well forward, people do spin them into the ground, usually with the CofG further aft (pax, bags etc) high power and crossed controls. If you're serious about teaching instructors / trainee pilots about spin awareness I'd recommend an aircraft certified in the aerobatic category and a spin entry with a fair amount of power, skidding and aileron to hold of bank. The Citabria does this extremely well. The classic power off, boot in rudder at the point of stall entry can be recovered at any point with minimal height loss. Use the climbing turn, rudder into the direction of turn and holding off bank with aileron you will lose at least 500' from the time it "lets go" until recovered. This entry also requires the correct recovery inputs - power off, centralise ailerons, identify the direction of the spin, apply opposite rudder, then progressively ease the stick forward until the rotation stops, then centralise the rudder and recover from the dive.