When I first started teaching, I created a few instructional tools to teach them basic sound synthesis. Each one, with the ability to record their sounds, demonstrated the basic synthesis technique well enough to be used in their assigned synthesis projects. These applications were installed on the studio lab computers.
Each are available for download and use on Mac OS and Windows.
This tool demonstrated Additive Synthesis with 6 oscillators.
This tool demonstrated Frequency Modulation. You can switch between Harmonicity and Modulating Frequency for the Modulating Oscillator.