As a student, you can enable/disable the webcam and mic via the video panel on the left.
Extra control options for teachers
As a teacher, you can enable/disable your webcam/mic and those of your students.
See what you can control here:
Enable/disable microphone (i.e. mute yourself or the student)
See who is speaking (changes green)
See snapshot of call quality (internet connection)
You may also choose to remove a student if you observe unacceptable behaviour or if their connection is too distracting for them to participate positively in the classroom.
Click the user's profile image on the bottom user bar to access Remove student option.