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It sounds like either you or the other person in the Online Classroom are not wearing headphones. Plugging headphones in and using them instead of speakers will usually solve any issues you have with echo-ey audio.
You might also get an echo if one of you is using Apple headphones with a Mac - the way the headphones are wired means that you’ll get some feedback. Using a different pair of headphones will fix that.
Another reason you might hear an echo is if the Online Classroom is open in more than one window - hunt down the offending tab and close that down to carry on with your lesson hopefully echo-free!
If this hasn't solved the issue, an alternative cause for an echo is an incorrectly configured Windows sound card - this is usually only the case if you are hearing yourself immediately as you speak, rather than with any delay. The solution to this is to make sure the microphone you are using is the default recording device on your computer or laptop.