By default Teams meetings are configured so that anyone can share their screen or mute other participants. I order to avoid this, a teacher needs to set the meeting options so that they are set as the only presenter. 

To set meeting options:

1) Go to your calendar, select your scheduled meeting and then select 'Meeting options'


2) On the Meeting Options screen, set 'Who can Present' to 'Only Me' and click Save. 

This will prevent students from muting teacher, or sharing their own screen. 

For further information on this feature and controlling students during meeting, please see this video: