Disclaimer: using earbuds with a microphone will provide very poor sound quality to class attendees. Since an external microphone will be critical to ensuring excellent sound quality and crispness, be sure to use headphones as a monitor.
For many classes, only one microphone may be needed (lessons on music theory, music history, digital production, trumpet). If only one microphone is needed, we recommend an external USB microphone with monitoring capabilities such as the AT2020 USB+ or Rode NT USB. Audio Technica makes great models with wonderful sound quality (example shown). While not cheap, the sound quality will be well worth the investment and its use will likely extend far beyond online classes. USB microphones are recognized by Macs and PC’s, and no additional software needed.
Some classes (guitar, piano, etc.) will require the use of multiple audio sources simultaneously. For acoustic instruments or amplifier cabinets, consider a separate instrument mic to optimize sound quality. If more than one audio source is needed (multiple mics, mic+instrument, mic+aux), this will likely require the use of an Audio interface.
An audio interface will give you the ability to connect professional microphones, instruments and other kinds of signals to a computer, and output a variety of signals as well (monitor, headphones, production software). Thankfully, Zoom will recognize an audio interface containing multiple sources without any additional software! Focusrite (example shown) and Personas are two excellent examples.
For more granular control of channel levels and your overall mix, consider a virtual mixer. Options are available online and run the gamut in pricing from free to hundreds of dollars. These tools will help you finely tune the volume and mix at any time and adjust as needed. And lastly, vocalists should always consider using windscreen!