Make sure to mute all participants and then to be absolutely sure, you can lock their mic so they cannot come onto the microphone. This is in the setting section below the user list (see below).
If you want to allow someone to talk you can unlock them manually by clicking on their microphone by their name. This is an absolutely critical step to ensure a session's audio quality.
Other Important Tips:
- Use Chrome which is shown to have the highest quality audio
- Use a headset, preferably a USB headset, which you can find for cheap at Amazon or most stores
- Don't run any other programs on your computer while you are leading the class to ensure nothing else is using bandwidth
- If all else fails, turn off your computer microphone, and call into the conference line with your phone
Again, if your internet connection speed is fluctuating or slow (less than 500kb/s, see this at speedtest.net) you should call in to the conference line provided in the chat box on the right of the classroom.