Why teach about AC?
It would be easy to leave AC as a slightly mysterious version of the direct currents that you deal with in simple DC experiments, and to suggest that detailed studies belong to later work in engineering. But AC is our standard form of supply. It is far more economical in distribution than DC because of the efficiency and simplicity of transformers.
Students are likely to be interested in the characteristics of AC. There are obvious ones (such as giving the same heating effect as direct current yet failing to move a DC ammeter visibly), and surprising ones involving phase differences. It is important to point out the differences between peak values, average values, and root mean square averages.
For elementary purposes, alternating current can be thought of as a current which is, at any instant, flowing in one direction or the other. As the alternating voltage changes direction, so does the current it pushes through a resistive circuit.