Any participant can "pin" another participant to the forefront of their screen, and only that attendee would be visible (i.e. Students can "pin" the teacher so that is the only participant they will see until they choose to un-pin them.) This only affects your view. Each participant can customize their own view.
To "pin" a specific participant, click the thumbtack / pin icon next the name in the participants list. This will cause that person's view to be the primary image displayed until they are un-pinned.