As the host or co-host in a meeting, you
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can manage your participants, including muting and unmuting participants to manage background noise and distractions. All participants can also mute or unmute themselves, unless the host has prevented them from unmuting.
Due to privacy and security reasons, the host cannot unmute other participants without their consent. The host can either use
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the Ask All to Unmute
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option, which will prompt every user to unmute themselves,
or can schedule the meeting with Request permission to unmute participants enabled,
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under Meeting Options.
Selecting this option will display a prompt when participants join the meeting. The prompt will ask for permission to allow the host to mute or unmute them. Permissions, once given, will apply in all meetings scheduled by the same host. 
If a student provides consent to be unmuted, you will have the option to unmute each person individually as needed from the participants list
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.
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If the students did not provide consent, you will see the regular Ask to Unmute / Mute options in the participants list.
Guide: https://support.zoom.us/hc/en-us/articles/203435537-Muting-and-unmuting-participants-in-a-meeting