It is dificult to explain Inertia without really understanding what it means.. Main Stream Physics define its properties very well.However According to Physisist Richard Feynman, Energy is not really understood by Scientists. Let alone gravitational Power!I am not go into General Relativity to Explain it.However ,what they call Inertia is really a gravity Power phenomena.. Gravity Power tends to lump(masses) objects together.. It takes application of power and use of energy to dislodge them.( in order word you need to eat enough wheaties so that you have enough power to move the object(overcoming inertia).It is easy to Test this Phenomena,by observing your passenger in the car as you make a sharp turn.Gravity Power will try to maintain direction and your car is using extra energy to change it. Your passenger will still travel in that direction and as a result ,relative to the car, he will be thrown to one side.. . You cannot explain Inertia without understanding Gravity.. The best thing to do for your students is tell the truth ,that you don't really understand it ,however ;this is what the effect of what science calls Inertia.