Echo can happen when your mic starts to pick up sound from the class or your row/table conversation.
If two people are watching in the same room, try using audio output from just one computer and input from just one mic. This helps to avoid overlapping which causes an echo.
It could also be the case that your speaker is too close to your microphone or is too loud and is being picked up by the microphone. You can fix this by wearing headphones or turning down your computer volume.
Finally, echo could be happening because of network delays. If someone's internet is delayed, it may cause delay and overlapping in the audio outputs. To see if this is the case, try visiting https://www.speedtest.net/ and checking if your speed meets our network requirements.